I'd go with 2 otos in the smaller tank - as you know, these fish eat algae and, as I hope you know, can be rather fragile. It's a good idea to under-stock a tank that contains otos and it's prefferable for the tank to over-grow with algae so the tank have plenty of fresh algae to eat than to get lots of otos so they clean out th tank but then have no access to fresh algae (which is actualy rather important in their diet if they are to remain healthy). Having said that, if the only reason you want otos is to clean out the tank, use an algae scraper instead. They are fragile, like I said, and not all that active either and, though they do provide a certain interest, are not usualy very noticeable and not worth buying if you don't actualy have an interest in them.
As for the flying fox. Flying foxes may not be as aggressive as RTBSs or rainbow sharks but they still don't like being with others of their own kind once they reach maturity. You CAN keep them in a large group to get round this problem but you need a larger tank. Stick to the single one you have and avoid adding anything that looks similar.
As for confusing it with other species, there are actualy 3 others - the siamese algae eater (a.k.a. siamese flying fox), the false siamese algae eater (a.k.a. false flying fox) and a fish that apparently has no common name, c. oblongus which is often found amongst FSAEs. Only the SAE does well kept in a small group, all the others are moderately aggressive towards others of their kind. All will eat algae but the SAE is the one with a reputation for being the very best cyprinid algae eater. If you do have a FF, you'll notice that its dorsal fin is vividly colored (whereas the other three have mostly transparent fins), it has 2 bold dark stripes running down the back and down the side (whereas the others only have one dark stripe on the side or a lighter one running along the back as well as the one on the side), dark-edged pelvic/ventral and anal fins (as opposed to transparent or yellow-ish in the other species) and over-all a much darker, brighter, more contrasting set of markings and coloration. Also, the line running along the sides extends into the caudal fin (but it also does so in the SAE).
