As of now, you can only get 1 transformation per character. (SSJ1 for sayians, Frieza form 2, Cell form 2, Evil Buu, Fused Piccolo, etc.)
Adult Gohan and other characters are not really in the game, but people do make models of those characters that you can put in to substitute the default models, but the attacks (and sometimes the sounds) remain the same. Check the Models/Skins section for current model projects, as well as information on where you can get some.