put it this way, the red tailed cat gets to around 15cm long, your tank is 30x15x12..... it's barely going to be able to turn around when it's fully grown. imagine keeping a dog in a room thats only twice as long as the dog itself.
clearly not right!
well as said above, if it's your first big tank then def check out the links in my sig, they'll give lots of advice and info.
if you want lots of fish then i'd go for minature species so you can fit quite a few shoals in then a couple of slightly larger feature fish.
i'd get a shoal of pgymy cories, two different shoals of small tetras, and a trio of something like apistogramma which have bags of personality and come in all sorts of pretty colour morphs.