I have two SAE in a 45G, and six in a 220G. They are extremely entertaining, as well as useful. They are a "big" fish even though small -- very fast, very very active. I would be inclined not to put more than a couple in a 55G unless they are the only/main fish.
I did a lot of reading about "true" SAE, and you should before buying. I've found very experienced fish stores with them mis-labeled, as well as the Pet* chains. I bought mine labeled "Flying Fox" but they are actually SAE -- the reverse happens probably more often. As best I can tell the key things to look for are the black stripe extending into the tail fin, clear not colored fins in general, and one pair of barbels on the mouth not two. The latter to me is the easiest to tell from the flying fox, but you have to be able to get a real good look (the second set is behind the mouth, more on the side, of the flying fox; Wikipedia's drawing is actually better to go by for the two mouths than the photos).
Mine are extremely aggressive eaters. They just cleared all the staghorn/hair algae I had, as well as the BBA (I did not have much of the latter, but it is gone). Only place they wont' eat is Jungle Val pu at the surface in the water flow.
BUT... they wills swarm any type of food added to the tank aggressively, whether flakes, bloodworms or (especially) algae tablets or veggies I am feeding the plecos. They seem to prefer hanging low and getting what falls through the surface feeders, but will go where ever the food is. Fast. They do not harm other fish going for it, but they can sure intimidate them, and get there first. It does not seem to keep them from grazing for algae, but if you have timid eaters they will make it harder to feed them.
Did I mention VERY fast. I actually bought 6 intending to move one or two to the small tank. An hour later, covered with water, and having tried two nets separately and together... I went out and bought two more for the small tank. I couldn't even come close to catching any of the first 6 in my 220G, at least not without destroying all the rocks and plants. Fast and skittish.
They do not really hurt my plants, but they do zip around through them and occasionally break loose leaves from crypts or wisteria. I think they do it by running into them, not from their eating habits -- their grazing is incredibly dainty, kind of buzzing along the leaf.
While fast I agree they are peaceful; they chase each other quite often, but I never see them bothering any other fish. While fast and active, they have shown no sign of being territorial. I have them with Angels (including a pair in one tank), serpae tetras, barbs, molleys, congo tetras, plecos, and rainbows. No fireworks with anyone.
I see no difference in behavior between the two, and the six, so I think all the advice "must get 5 or more, or just one" is questionable. Mine are pretty big (maybe 4.5-5") so I think into adulthood, and seem well adjusted both as 2 and as 6. The two appear to be a pair (based on body shape and some guesswork), so that may help.
All in all the best value in fish I bought -- entertaining, cleans algae... what's not to like.
Oh... be careful... they JUMP. I've never had them jump in the display tanks, but while acclimating them I had them in about 3" of water, dripping into to a bucket, with about 8" of bucket above. I came in later and 3 had jumped out and died. Amazingly high jump given the shallow water. They seem happiest in the middle/bottom of the tank now, but be careful if they get spooked or while acclimating.
Bump:
Would three disturb german blue rams? As I said before, I have a 55g so i don't want it too crowded, so I'm debating whether to get just 1 or 3.
Personally I would get two. They seem very social with their own kind, and I think they will be more entertaining. Three probably won't hurt, but they take up a lot of space just from being so fast. Other than trying to get in the front of the line for food they won't disturb your Rams.
I asked my LFS if they were SURE they are SAEs. They seem to know what I'm talking about and I will be inspecting them myself before buying. She said they will be available on monday, so I'm trying to figure out what to do.
Frankly I would not trust them. Not because they want to fool you, but I really think there's a lot of honest misinformation floating out there. A very, very reliable guy sells them as flying fox, because that's what his distributor told them.
You may already have these but in case not:
Well researched descriptive article
Wikipedia article - look at the drawing
Wilipedia flying fox - see drawing