Is it necessary to get all the oneshot off before throwing in the powder? Will the residue from one shot interact with the powder?
No. One Shot dissolves, and doesn't leave any residue on or in the case. It will not contaminate your powder. Now, this being said, if you really get carried away with the One Shot, and try to throw powder right after you use it, you might contaminate your powder.
My normal procedure is to tumble fired cases first, for about two or three hours. I then inspect for defects, put in loading blocks, spray with One Shot, and resize/decap. I will throw them back in the tumbler for fifteen minutes or so afterwards just as a precaution against any oils or solvents that I may have gotten onto/into the case. After they are tumbled for fifteen or so minutes, I continue on with my loading process (prime, powder, bullet, etc).