A SMB should do fine with larger sunfish, though take my word that a 125 will soon be too small for anything but the SMB by itself.
I have a 125 with a 9" LMB around 8 longear sunfish, four 4 inch or so rock bass, a 7 or 8 inch bullhead, a 5 1/2 inch green sunfish, and a 4 inch madtom, it's already starting to seem pretty small for them.
SMB seem to like better water quality than LMB and sunfish and bass put off a fairly large amount of waste and eat a lot. A 125g will get pretty nasty with a large SMB and several sunfish.
Also, if your SMB does get max size, around 2 feet, that tank won't even really be big enough for him physically.
As far as the water quality factor though, I don't think you'll be happy with a 125 when your SMB and sunfish are big because you will either have to do lots of water changes or it will get pretty grungy.
Of course money factors in, but if possible I wouldn't go with under a 220, not just because of the fish but I think you would be much happier with the lower maintanance factor.
As a suggestion I would also go with barebottom. Barebottom can look really nice if you have driftwood and stuff, and with sunfish/bass substrate gets really really nasty quickly, depending on your fish load.
I just got done taking half the gravel out of my native tank because I'm tired of cleaning it, it gets kind of tedious. Then again I have 9 other tanks to maintain, but even by itself it would be kind of annoying.
Another option is grow them out in a 125g then sell that tank and get a larger one. That way you'll also be able to guage if they will even need a larger tank.
Just figured I would give you a heads up though, 125g may end up way too small.