>As I understand what you are doing is to regress our hived bees to a wild state, in so far as their cell size, which results in smaller bees and the end result is less or possible no varro mites.
The end result is significantly less Varroa mites. I think no Varroa mites is not possible. But only a few mites aren't a problem.
>My first question is can this be done in one step
The only problem in getting it done in one step is getting the artificially enlarged bees to drawn natural sized comb. They are quite willing to use it.
I have successfully done it in one step by wax dipping PermaComb (fully drawn plastic comb) so that the cell size is 4.95mm.
>or does it take several requeens , can a large queen lay in the small cells.
Usually the large queens prefer to lay in small cells. It's not requeening that is required but rebuilding all the comb to the size the larger bees want and then after raising bees on that smaller comb (usually averaging about 5.1mm for worker brood instead of the 5.4mm that their "parent" were raised on) then those 5.1mm bees will build an average of about 4.9mm cells. Those bees from that will usually build all the way down to 4.4mm cells but most WORKER cells will be in the 4.8mm range.
>I recently aquired a swarm ... and the bees are noticible smaller than my Itallians
Put them on 4.9mm foundation and you have regressed bees in one shot.
Piece of cake. Or put them on foundationless. Your choice.
The whole catch 22 is that the larger bees won't build 4.9mm cells and you can't get smaller bees without smaller comb. The only reason it works is that they WILL build smallER cells. If you have small bees and let them, they will build the small cells. If you have the small cells and let them even the large bees will raise small bees.
You have small bees. You're in business.