id try to stick with fish from the same general area. angels come from the amazon south america region.
so finding tetras that fit with angels would be good. no fin nippers as angels will gett fin nipped. maybe go with neons, glo lites , black neons blue neons, head and tail lite tetra, lemon tetras, red eye tetra, stay away from more agressive tetras like serpaes and black skirt tetras. also is your tank planted what kind of substrate. most tetras and angels as well prefer planted well shaded tanks. as for bottom feeders cory doras come from the same general region as tetras and angels and require much the same water parameters and conditions. there are a plethora of cory's to choos from in a variety of sizes. they also prefer snad to gravel.
as for algae eaters ottos do an excellent job and do well in odd numbers like 3. they also come from south america amazon area.
with a shoal of a few different tetras and a shoal of corys it should add to your load.
for example
6 red eyed tetra 12 inches of fish
8 neons 8 inches of fish
8 glo lites 8 inches of fish
6 juli corys 12 inches of fish
3 ottos 5 inches of fish
=45 inches of fish which leves you with close to ten inches of fish left relegated to yuour existeng load of angels and rasboras.