Okay, I'm still new to kilting myself, but these are my opinions.
Since it is an evening affair, I myself would avoid T's. A polo, or my preference, a nice long sleeve shirt. Somehow I just don't get hot, so I have no problem wearing long sleeves, beside you can roll them if you do get warm.
For evening I would go with black belt, sporran, and shoes. Perhaps the trad. sgian. From what I gleaned from above, you wanted a cleaner look, but not formal. A polo would be good; I just don't seem to like them myself. I like an informal long sleeve with sleeves rolled up when needed. One last thought; for me, boots are not for evening occasions.