I have one of these Ruby/Rainbow sharks. Have had him since very little and he's now over 4" long (had him about a year). When I first got him he was very peaceful for a few weeks but then the aggression started to show and he earned himself the appropriate nickname of Billy the Bully! I had to buy another tank to set up and eventually moved him into that with his female guppy friend (she was the ONLY fish he didn't bully - he hated the beautiful male guppies but the boring female he seemed quite happy with LOL).
I then did a bit of further rearranging and for the last 6 months or so he has been in the 3ft tank with my two short-bodied pink convicts. He wouldn't dare bully them! In fact, he's scared of them, if anything, even though they are smaller.
I wouldn't consider Rainbow Sharks good community tank members as they are just so territorial and will chase other fish away from their patch and the other fish end up stressed out a lot of the time. You might get lucky and end up with one that is a bit more placid than mine - but if not, do you have a spare tank to do some re-shuffling if need be?
Athena