lol ya i saw your other thread, which is why i say you're not gonna have ANY bioload issues
if you want a manderin...definately get that fuge goin asap. The manderins need tons of pods to survive. Some can get them to take other meaty foods, but i dont think too many people have luck with that..could be wrong tho. Your amount of live rock will probably help w/the pods...but i'd still do a fuge. That'd be an amazingly well set up tank haha.
For the clams, i guess they could do ok in a 20 gallon, but i think they've gotta be in the sand, and they get huge, so your tank may be a bit small, unless you were to get rid of them whenever they get big. And if you want those, def. go for metal halides (if you havent already got that) or maybe some t5's.
Oh, and i didnt think you personally were going to put that tang in a 20 lol. I was just makin sure that you didnt copy that guy. Replies on here are mis-inturpreted MUCH too often, because its kinda hard to type in the right tone of voice, you know? (and it always seems that mine are the ones missread

) Its all good tho.
I have a sixline in my 55. I dont know if those are considered flashers, (i dont think so) but its an awsome fish...only i wouldnt put one in a 20 because they're really active fish. Mine is constantly weaving in my rockwork going back and fourth in the tank, so i'd say 30 would have to be the min for them....unless your tank is like 3 ft. long and 20 gallons haha.
hth
mike