What kind of clowns do you have? If you really wanted to do a eel i would get rid of the canister filter and do a sump/fuge set up. You could use a used 29 for the sump/fuge. Than get a asm-g2 or a g3 skimmer. You could get a SFE for your 90.
What kind of aggressive fish is your friend thinking of keeping? This will help deturmin the tank size needed. Have them make a list of fish that they are considering. Good filltration will be needed for aggressive. Aggressive fish are heavy on the bio load.Depending on the fish keeped cleaning...
I will depend on the coral beauty not all are reef safe. I just had to take mine out of my reef tank due to nipping at some corals. I have read alot of poasts where people have not had a problem but others have had problems so it will depend on the fish.
It dosent hurt to have some in the tank as long as you dont have an boat load of them or any that are extreemly huge.. You could always get a Arrow crab they eat them. Also a six line wrasse eats them as well. I have a arrow crab in my reef tank.
I dont think he can see himself he is just getting use to the new tank. Post a pic when you get the chance and by the way congrats on the new eel. I have one of them also.
Skip the silversides feed the fresh shrimp uncooked with shell on. you can try oysters and mussels squid,krill. just offer as many different things for him to eat.