If they are all well established snakes I would skip feeding and treat for mites right away. I would also skip Mite Off and instead use Reptile Spray. Mite Off has not had great reviews on here but many of us have used Reptile Spray with great success. Its sold at Petco for $8.
Treating for mites is going to require moving the snakes alot and then placing them in homes that may not be their norm. If your set on feeding them I would say wait two days but that is just two more days the mites have to get more established.