am i in risk of overstocking?

Cories and tetras need at least 6 individuals. Add a couple more cories of the same species, some more tetras of the same species and one more angel if you want. You should be able to add a pair of guppies or a male and two females.

All depends on the betta really, some are very aggressive and can't be kept with anything, some can be kept with other fish. Its really a trial and error experiment, I would only add the betta when you would be home for a couple of days to observe, and have a back up plan/tank for the betta if it doesn't work out

I agree that you should first add more cory cats and more tetras of the same kinds you already have. It is very neat to see their natural behaviors and they do this best in groups of at least 5.

From what I have read, it can be hard to introduce new angels so make sure there is a lot of cover for them.

A betta does not sound like it would do well in your tank. It could be nipped by the angels (they are cichlids, after all) not to mention nipping the angels in return. I would not add guppies to a tank with angels because they will most likely be eaten. Also, bettas and guppies are usually not compatible. I realize this can vary fish by fish.

i would say put the betta in there and see how it is. i put a male in my tank with the other fish and he does just fine hes not aggressive what so ever.