2-4 clowns in the same batch may be troublesome. Clowns can be some of the most aggressive fish. If you plan to put a pair in each tank, I would separate them and not do all 4 at once. Say order pair 1 for wife for first round of QT then pair 2 for second round of QT.
Tangs would be best to be added to DT at same time if possible. Or at least in groups. I don't know what you plan to have but they can be very territorial in the DT. I personally like having an Acanthurus in each round of QT, for my tangometer (very succeptable to parasite affliction, so it's a good way to be sure that the batch of fish is indeed "clean" at the end of treatment). I have a large desjardini Sailfin that is 6". He is my peacekeeper in DT. That's another way you can keep multiple tangs and add them in small groups. I try to keep the others around 4" or a little less and so far so good. They bicker a little here and there. Never for long though. That could change at any time.
Treating "like" fish at same time works for some less agressive species, but not others. Say you wanted two yellow tangs, one for each tank. I would not recommend QTing them in same batch as one will most likely kill the other. Not a guarantee, but it is possible. If you want 3 or 4 firefish, no problem to QT them together. Just ask, if you question it. There are tons of people on here that have experience with all kinds of combinations. You can probably find someone that keeps two yellow tangs together in same DT, but it would be a rarity. They might be fine for a couple years then one day one fish decides he doesn't like the other any longer.