Water change question

As far as stocking,the type of filtration you have will play a factor in determining levels. Check this out:

http://www.aqadvisor.com/

I would check the ammonia, nitrite, nitrate levels.
if the tank is fully cycled you may see a mini spike/cycle.

typically, cory's do better in schools. (6)

I'd go ahead and do the water change by Tuesday even if water parameters look fine. It's a good idea to do that first weekly water change regardless. It can also help keep the nitrates from creeping up on you.

I would not risk an angelfish in this tank. I have seen many angels easily eat small fish like rasbora and tetra, but in reverse the small fish can pick on angels especially when one is alone. Angels are after all shoaling fish, living in small groups (five or more, maybe four, but never two or three).

I too would increase the corys to 5 or 6, the same species if you like them, or you can add other species. The more corys the better, for them, and probably you as well, as they are very social. I would also increase the rasbora; I am assuming this is the common Harlequin (Trigonostigma heteromorpha), and this as well as the two very similar-looking species in this genus are better with higher numbers. I would aim for 8-9, up to 12 in your situation.

And yes, there is sufficient space for other fish if they are carefully choosen, bearing in mind the sedate nature of what you now have. An active swimmer like any danio or barb would disrupt the tank.

Byron.