If you trade one IR player for another, when the trade goes through will you have to drop another player 1st (or does Yahoo allow you to Temporarily go over the roster cap until you can assign the new player to your IR?)?
No. Your IR player must be on your active roster at the time that the trade is processed, otherwise the trade won't go through. That would prevent your team from being over the roster cap at any time.
In order for your IR player to be on your active roster, you would either have to drop a player or swap him out for another player who also happens to have IR status.