Hi, I have three true SAE's in my Rio240 and I'm actually considering re-homing two of them. Yes they should be in a group as they are natural shoalers but they grow HUGE. They are now bigger than my ancistrus catfish and the 4ft tank is just not big enough for them.
All that said they are fantastic algae eaters - especially if you have black beard algae. Mine are very greedy and bicker over food. I can give you a brief run down on how to tell them apart from the similar flying fox
Firstly look at the tail. Does the black lateral line extend into it and does is taper off. If the lateral line stops at the base of the tail it's a flying fox. If the lateral line gets considerably wider at the fork of the tail it's a false siamensis.
Next look at the lateral line running down the length of the body. Does it look slightly jagged at the edges or does it look smooth and is there a gold line running underneath. A smooth lateral line with a gold edge is another sign of it being a flying fox and not a true SAE.
Thirdly, look to the mouth (if you can, if they are very young it's not easy to spot) The true SAE only has two sets of barbels, the flying fox has 4.
The flying fox also sometimes has a red tip to it's fins so look for that too. The flying fox needs to be kept alone as, Far_King says, it can be aggressive and will kill any others of it's own kind. It's also not a good algae eater and won't touch BBA.
I hope both my info and the link Far_King gave are useful. If you take your time checking out the fish and make sure you are buying a true SAE you'll be fine - provided you have the space that is
Here's a photo of my SAE's so you be prepared for how huge they grow