30L is about 8G right? platties are fine with bettas but i would be careful not to overstock your tank. If you put in just a pair of platties or something (maybe a trio) it would probably work
guppies and bettas don't go the guppies usually have a very fancy tail that the betta sees as competition (and could possibly think of it as another betta), and cuase him to attack the guppies, which is a bad thing because chances are both sides will be hurt.
so with the 1 male and 5 cories you already have 6 fish, i'd suggest you just get a pair of platties to top it off and you should probably stop there for now