I can easily answer this question as I currently have a 55 gallon with angelfish currently and schooling fish! The recommended amount is 1 per ten gallons so really adult size you would want only at most 6. However if your hoping they will breed then you can "overstock" with baby and juvenile fish up to 10-12 (I currently have 12 juvies) and then once they start pairing up you can take some out.
As for schooling fish, I recommend blood fin tetras and rummynose tetras. Rummynose are an uncommon tetra that looks cool with bright red noses and are one of the best schooling tetra. The blood fins are probably the second best schooling terra behind rummynose and occasionally they will school together which is really cool. I have 8 blood fins and 7 rummynose for a total of 15 and they'll school with their own groups or occasionally I'll find a blood fin with 3 rummynose or some kind of combo like that. That's what I would recommend IME. If you would like to see pictures of this setup you can go to my profile and look under my picture album titled freshwater 55 gallon angelfish tank and you'll see all these fish coexisting together. I hope this helps you out!