So yes tangs are a hot topic for sure… Ive got a six foot with a purple, yellow, blue hippo, and a yellow eye kole. Purple is close to being full grown, yellow is about 4 inches, blue and bristletooth are about 2-3 inches. And no Ive never seen clownfish outside of maroons get 8 inches in the tank.. my adult female is 3 maybe 4 inches. I had my purple in my 70 gallon (about 4 and a half feet) for a month before I could put fish in my six foot and he was very unhappy in there. World of difference. The only small tang I have is my bristletooth, the other three Im expecting to reach full size and calculated for that to be safe. We all know they dont reach ocean size in tanks, but I still plan my stock around that. I also listen to others on here as far as advice for what to add and what not to add. Its your tank and your fish! I just suggest researching these fish. Purple, gem and black are all in the same family and are aggressive and if they dont have a lot of room that could be an issue. I had issues with my purple and yellow fighting in my 6 foot for a week. They still squabble some but its died down. Having an established purple I would not add the other two in that short of tank in my opinion from what I saw with adding my yellow. But if you do want to still do it, look into tang decoys and have an acclimation box ready to catch and put your tangs in to let the others settle if they become aggressive. Thats just my two cents from my experiences .