yeh only add one angel. they get quite large. i have a tank the same size and my 20 cent size angels are now about palm size after 6 months and they are still growing quickly. along with my two gouramis the tank looks pretty full. you will not have enough room for anymore than 2. besides if you cramp them up they might get a bit territorial. angels are best in pairs in the tank size you got.
how long has your angel been solo? if he's been solo too long (more than a few months) he may nip the new angel. if so turn off the light, rearrange the tank and turn the light back on. he'll need to set up a new territory and they should get along better.
ummm yeh from what i hear and from the remains of the neon tetras in my friends tank, when your angel gets bigger they will become snacks. how about removing the tetras and adding a few more corydoras. they like to be in little groups of about 5. they'll snuffle about together. good luck
PS about the angels size, i'm sure they could get bigger in a huge tank but i agree that in a 30 gallon tank an angel won't get much bigger than say 7inches(excluding tail) but that is still a v large fish for a 30 gallon in my oppinion. thats based on what i've seen in friends, family and a very large display tank at my lfs (7ftx3ft tank with about 7 angels and other assorted fish)