Well, three platies can turn into 30 pretty quickly, so having them in the tank means not having much else, honestly.
If you were doing an albino-cory only tank, I would say 4 or 5 would be okay. However, when you start adding fish other than these, you take away from how many cories you can safely keep.
That being said, be aware that cories are schooling fish and like to be kept in groups of at least 3. So, to remedy the problem of multiplying platies (without having to cull young, which is sad at best), I would suggest finding a fish that doesn't bear live young at all, such as a small tetra species or a pair of dwarf gouramis.
I'm sure a dojo would be fine, but I've never had one, so someone else will have to chime in on that.
Make sure you read up on the cycling process before you add anything to your tank. Water quality is essential to being able to keep a healthy, thriving fishy community.