I would definately recommend getting as many of the same cory rather than 1 or 2 of a few different ones. Yes they will shoal togther but this is out of need not want. They need the security of a group, so if the only corys in there are a little different to them it's a case of well it's better being with them then alone, wheras given the chance they stick to their own kind IME.
Originally I had 2 gold corys that came with a secondhand tank, I added 2 bronze cory(couldn't find any golds) and they all shoaled together happily, then I added another 2 bronze and the bronze started to breed. Evetually I added a trio of pandas and they would ocassionally shoal together but the pandas would quite often be in a group together and the bronze would with the golds splitting their time between both groups, but the golds always stick togther, where you found one the other would always dart over a.s.a.p.
The pandas got their own tank and their number bumped up, I got some trillis(group of 5) and they definately stick to themselves, they very rarely shoal with the golds / bronze. I then got 3 more golds and even though they mingle sometimes the bronze and the golds tend to stick to their own little groups.
I also noticed a change in behaviour in my pandas & golds, when the pandas got their own tank and more were added they became alot more active and I believe they are getting ready to breed. The original 2 golds were always very very shy and would dart out for food and then hide again, wheras since adding 3 more they are alot more active and they are out and about all the time now even when I'm next to the tank.
So from my experience with 4 different types of cory I would most definately advise getting a larger group of just bronze for the one you have already rather then different ones.