The Hattori "Forum" knive is really too much for oilstones. You really should make the switch to waterstones.
Which stones to get depends on a lot of factors like price and ease of use, and also on where you peg what you think is the most important stone. For most people that's either the final polishing stone or the stone they use most often to "pull a wire," that is, start the actual sharpening process.
If the knives don't get too dull or damaged, something in the 700# to 2000# range is a good choice as that first sharpening stone. In your case, it's probably adequate as your coarsest stone -- at least for awhile.
Given the nature of the FH, you'll want to finish with at least some polish. Somewhere in the neighborhood of 4000# to 6000#. You can live with it as a final polish, and it's not an impossibly long step from the sharpening stone.
If you're already a competent sharpener, a combi-stone isn't necessarily a good choice. They wear out pretty quickly, one side faster than the other. They're outstanding for beginners though. Knowledge keeps pace with wear, and by the time the stone is ready to be replaced, the sharpener knows enough to make informed choices about better stones.
There have been some changes in sharpening stones over the past few years. Both in abrasive and binder. As a rule, modern stones require less flattening and soaking than their older counterparts. But they're still waterstones, and therefore a PITA.
There are several ways to flatten. You'll have to choose one right off the bat. You cannot sharpen with waterstones and not flatten. You may get away with three or four sharpenings on very fine stones between flattenings. But best practice is to flatten the sharpening stone before every sharpening. Probably necessary as well.
The trend with high-end knife hobbyists seems to be flattening with a coarse diamond stone -- usually a DMT XXC. IMO, they're too small to do a good flattening. "Flattening stones" are also popular. When I used waterstones, I flattened on sandpaper, but understand drywall screen is a better choice. If and when I go back to waterstones, I'll flatten with screen.
Up until a few months ago, Shapton GS ("glass stones") were the hot choice on the knife boards; but the trend has shifted away from GS in general and towards more conventionally bound stones. GS are generally excellent stones. At every grit, they're very fast AND give a lot of polish. On top of that they're among the easiest stones to maintain. On the other hand, maintenance can be expensive -- especially if you go the Shapton recommended route; they work best in the Shapton holder -- expensive; and they provide lousy feedback.
When it comes to feedback, Shapton Pros are off and on. Some of them are excellent and some lousy. I really like the SP 1500# and 5000# for your situation, but the 5000# is just horrible for feedback. One of the best stones I've ever used for results and one of the worst for feel. Sharpening on the 5000# feels like your running the knife over jello on granite.
Naniwa makes too high end stone series -- Super Stone and Chosera. You can't afford Chosera so just forget it. SS are great for feedback. In fact, maybe too great. Throughout the range, SS not only feel soft, it's easy to cut into them if you're not paying attention or wobble the knife on edge-forward strokes. Attention deficit aside, it's a big problem if you sharpen with a lot of pressure on the knife -- especially if you have a high stroke rate. Considering that you're coming off an 11" tri-hone, it's highly unlikely you do either. But you should know.
There's another problem with the SS 5000# in particular. After it's been used a few times, the surface develops a crazed appearance. It doesn't effect the stone's performance though. After everything's said and done, the SS 5000# is the best choice for most people who want a quality stone at this grit level. Probably your best choice too.
At the 1000ish level, the best choices are Sigma Power 1000#, Bester 700#, 1000#, or 1200#, Shapton GS 1000#, Shapton Pro 1000#, Shapton 1500#, and Naniwa SS 1000#.
If you're considering building a core set of FH or similar stainless knives, an ultimate "mix and match" set would look something like this:
Naniwa Omura (extra coarse)
Sigma Power 1,000
Naniwa Super Stone 5,000 (unmounted)
Sigma Power 8,000
Hand American sharpening kit (including felt pad for deburring and two leather strops -- one for chromium dioxide, the other for diamond)
Idahone 12" fine honing rod.
You'd start with the 1000 and 5000, then add the others as need dictated. The Sigma Power 1000 is sufficiently aggressive to do some thinning and light profiling. Almost as aggressive as a Bester 500.
I called the set "ultimate." Well, maybe ultimate in some senses. If you're seriously into sharpening you'd want a few more stones to fill in the gaps in the grits. However, there's such a thing as too many stones. For one thing they increase the probability of error. And for another, while more stones may mean less time on any given stone, it doesn't make sharpening much (if any) quicker, and it drastically bumps maintenance time. If you're a hobbyist sharpener that might be a pleasure. But for most us -- ugh.
There are more expensive, better stones -- like the Chosera series. There are even a few stones at the same price which might give better results but for one reason or another are a pain to use (like the Shapton Pro 5000#).
There are also stones which work somewhat differently. For instance a manmade aoto (like Nonpareil or Naniwa) is an excellent intermediate stone on the way to a final polish, but isn't a polishing stone itself. It works best by building up a lot of slurry at the stone surface (i.e., "mud"). It's great if that's how you like to sharpen. Some do, but a lot of people don't. The stone also dishes easily considering the grit, and needs a lot of flattening.
If you wanted to achieve the same intermediate effect without the mud and maintenance, you might choose a Sigma Power 2000 instead of a 5000. It would leave you with something very close to a King or Norton 4000 level finish. Good enough to go on to a fine polish -- especially if it were a fast stone like the Sigma Power 8000. But again: The 2000 is not a polishing stone, it will give you a good intermediate finish -- good for butcher work for instance -- but is really more of a stepping stone to a polish.
Some folks just throw up their hands and go with something that appears simple (at least in concept) like the Shapton GS series. Not a bad choice at all. They're excellent stones.
If money's an object, you might want to start with the Naniwa Super Stone series that come glued to their own "sharpening stands." Actually, the stands suck because they flex and can slide. But the stones can be had very reasonably -- as cheaply as Nortons. And if you don't push hard or move the knife too fast, which you really shouldn't with Naniwas anyway, the stands are adequate. You can get a 400, 1,000, and 5,000 for under $100 from Tools for Working Wood -- and my bet is that this makes the most sense for you.
Hope this helps,
BDL
scared me poopyless when you sliced through that onion 
but yeh, next time we can do more vegetable cutting 
