About Kenny Thomas, think of it this way. He's undersized for PF and not very athletic for a SF but despite his positional problems, he's a solid player. Consistent double digit rebounder and scorer. Good jumpshot out to the wing (though lacking three point range). A capable passer. And the fact that he's signed long term isn't necessarily a negative, depending on what team you're looking at. It's bad for us because he doesn't really fit our needs all that well (ie he's not going to give us much of a defensive presence at PF), but it could be good for a team that needs a SF with some rebounding punch and wouldn't mind having him stick around. And for a starting forward who grabs 10+ rebounds a game when given the minutes, his salary is actually quite reasonable.
My reasons for wanting to trade him are pretty much the same as yours VF. He's not giving us what we need in a starting PF (given the Bibby, Peja, Miller core which is good pass and shoot offense but no post defense whatsoever) and he's probably not going to be happy with a backup role. Not to mention we don't need to be paying a backup forward that kind of money unless he's big and he's got a mean streak. Neither of which is the case here. A solid player, just not right for this current system.
Skinner is a good veteran forward off the bench who'll grab rebounds and play tough around the rim. Very useful. Corless' value is decreasing as he's getting on in years, but he's only two years removed from a 6th man of the year award now. Given a regular role, I think he could be a nice backup SF off the bench, though not really a game changing one so I'm kindof ambivalent on whether we keep him or not. His contract is big enough to trade up for a new starting PF when combined with someone else's, but that doesn't necessarily mean there's a team out there willing to make that trade.
Really I don't have a big problem with us keeping any of those guys because they're useful players. But none of them is a legitimate #1 option for a playoff team, and having three backup forwards with similar games is overkill. We had a problem with depth before. Now we've got depth at PF, but no legitimate starter. If it were up to me, I'd hang onto Skinner as a backup PF, move KT and Corliss, and draft a PF. If the trade doesn't bring in a starting PF, I'd start the rookie - whoever that turns out to be - and use our MLE money on Evans and another SG.