First consideration is your water parameters (pH and hardness). While many fish can adapt to some extent, there are those that can't and there are some like livebearers that simply always do better in basic (harder) water.
Have a read of the "sticky" on fish compatibility, there's a lot of info in it, at the head of this section, but here's a direct link:
On the corydoras, they are shoaling fish that must be in a group to be healthy (shoaling fish will be under constant stress on their own or even if just 2); 3+ is the minimum for any of the corys. Same holds for the loaches, another suitable bottom fish; some of these can get large though, so only a few species are suitable for a 30g. Single fish for the bottom do exist, there are some of the plecos that remain 3-4 inches (avoid those that get 18-24 inches plus). Again, consideration must be given to water parameters, and also filter flow as some need current, others do not. Compatibility has many aspects.
For the "larger" fish, some dwarf cichlids might do well, depending upon water again. Or gourami. Angels need more than a 45g in my opinion. Have a look at our fish profiles section for some ideas; any questions that arise from those we will certainly answer.