hey!
wow, you want to send me that clam?
hehe.
i would say leave the clowns in the 20... if you want to keep up with another tank... if you're just feeling guilty about getting rid of them, don't. find a local reefer to take them and give them a home, rather than the LFS.
as for the tang, how well established are the other two? i would think if you upped to a 180 they'd probably be ok, but tangs are just hit and miss as far as getting along. you'd have to introduce them all at the same time to the new tank, which could be tricky. other than blind luck, the best successes i've seen were all the same size tangs at the same time.
ultimately, it's up to you, but it really depends on how long you've had the 125 up, how badly you want to upgrade, and if it's not an impulse upgrade just to keep one more fish. if it IS an impulse, i'd just find a new home for the blue guy too.
going from a 125 to a 180 will be a major PITA.
but i always encourage upgrades! i want a 90!!! (but the roomies said no).