To get less agression, you rearrange the tank, and by doing so, it sometimes can reset terroritories, which by buying new stuff you essentially did. Angels are unpredictable IMO (seen real agressive and/or territorial ones and real timid ones), as well as fish raised solo for a bit, so hopefully it with the rosy barbs will work allright. Also as far as introducing other fish for future reference you should do it after a feeding in the dark as so they slip in unnoticed and have a chance to slightly settle (as well as quarentine them beforehand for preventive maintence, as to not spread possible diseases they may have). As far if I were you I would upgrade to something higher when it gets larger like a 20 high or 29, as they really benefit from the added height, since they are more of that then length.
Sorry for the overkill, and although I don't raise angels, I keep quite a bit of fish myself (over 500 gallons and counting) so I figured I would try and help with some problems I see that might arise.
Good luck and if you ever have any other fish related questions, let me know.