Luckily, the shoal grass isn't as fast growing as the oar grass. The latter is the short bladed grass you see filling 90% or more of my tank. In a bigger tank, it should be more easily controlled--or at least you have more time to slack before trimming. Also, shoal grass produces runners above the substrate level (and then anchors down in spots), so it is easy to watch its activity. Oar grass looks like it is doing nothing and/or dying, slowly making a large under-substrate network without you even realizing it. Then it sprouts up and surprises you. In all, though, I think that unless you have a system that specifically targets the grasses' needs (or exploits their strengths), I don't think it will be anywhere close to as invasive as you think. In my old 40g reef, which is when I first got the grasses, it grew very slowly. I had it for months and it grew about 6" in a few directions--probably 15-20 sprouts in total. They can do fine in such tanks, but need something near the roots for food, which I didn't do. Here's a pic of it in that system:
For care, it helps to have an established sand bed and a quiet area to allow the grass to settle and root. The bed doesn't have to be deep--about 3" or a bit less is fine for shoal grass and oar grass. Any kind of sand and/or mud combination works (mud is not necessary). It can help to punch a few fish food pellets around where you'll be planting the grass, which will provide a bit of extra nitrogen that the plants will need. They should root relatively quickly. If your system is on the lean side in terms of nutrients and feeding, it may grow more slowly than you'd like, but I think if you target the roots with fertilizer (food, etc.) you'll be okay. Once the grasses have settled, you'll want to give them decent water movement--enough to move the blades around. High flow will not be an issue for rooted plants--they tend to do even better with it. Light needs are reasonable for this species--bright, but not ridiculous. Part of the reason for the intense growth in my tank is the use of 10K 250w halide very close to the surface, but it isn't 100% necessary. T5s, especially decent lamps and/or overdriven, should work fine. The grasses do best with a warmer light color temp, but will grow with bluer light, but more slowly. Beyond those basics, just keep algae from smothering any blades or anything like that. Most herbivorous fish will avoid these, too, with the exception of oar grass (some blennies seem to eat it to an extent, so I'd imagine tangs would, as well). Also, as leaves get algae on them, you can simply pluck them. It is good to pluck leaves, anyway, as it makes them healthier in the long run, just like terrestrial plants. They also get root bound if growth gets really dense. Easy to prevent, though, especially in bigger spaces. That's about all that I can think of. You can add iron, if you'd like, but I don't bother. Dead leaves that drop off by themselves are neutrally buoyant, so they will float around and mostly catch on powerheads. The shoal grass is easier to remove than the oar grass, fortunately. Breakdown into detritus is pretty fast with the leaves, but nutrient release is fairly slow. It is an entirely different class of detritus (more like "true" detritus like you'd find in the ocean).
Max, I just got your PM. Sorry I didn't catch it earlier. I do still have the grass, but it is going to be in a bit rougher shape. Are you able to pick it up tomorrow?
Oh, and Drew, I did get a few pieces of shoal grass this last time around (though I don't pull it up regularly, because it grows more slowly), but they barely had any rhizome. In fact, almost none at all after I started examining them. Chances of it surviving is probably not good. The oar grass I pulled up is in excellent shape, though, and there is a LOT of it. If you are still interested mostly in the shoal grass, I will set some aside for you the next time I trim, which will likely be within the next month or so. Most of what I'll have will be oar grass, though.