Should/can I add anything else to my tank? Any advice?
They may be small right now, but you have to consider what it will be like when all your fish are adult size. Angels can get larger than 6 inches and can be quite territorial. I have one (she's 6 years old) in my 36g and can't image fitting another.
I don't think you're horribly overstocked, but if it were me, the only thing I would add is a couple more corys because they like groups of at least 5-6.
20 gallon high-tech planted