Yeah, that's a very heavy load for a 30, especially if you have (or plan on) corals..... keeping nitrates down will be a nightmare.
I'd definitely stay away from the mandarin. The cardinals like schools, and they can get suprisingly large..... a little too big for a 30, IMO
. I think a pair of clowns, a six-line wrasse, and maybe a goby or firefish would be a good stock for a tank that size.
I've got a 28, and I only have an ocellaris and a six-line in mine..... Might
get a third down the road, but that would be the limit for me.