Thanks, all.
Here's what I've done and my plan:
I put the each girl in the pool with my plate/wing and a single, after spending some time fitting the harness to them, and jury-rigging a single tank adaper from an old single tank backpack (at least 25 yrs old) I had. Obviously, nothing fit properly - both complained about the plate digging in. It was just too long for them and the wing was way too big for a single.
No problem with the rig shifting more than expected with misfit plate. Excellent trim/bouyancy control. No problem inflating/deflating. No surprise to me, it worked really well.
I'm going to order a FredT short plate and do the same experiment before ordering the proper sized wings for them. If it doesn't work out, then I'll either mothball it or ebay it, and then I'll start looking at BC's.
Now time to start thinking about regs. I have a mental block re: ScubaPro, so probably won't go there.
Thanks again to all of you
Brad