I used to do this back in the day.You don’t need to attach to plugs. You could also use rubble
I wouldn’t cut them. When I had this issue on a prior tank I was able to peel them off. Like peeling off a sticker
Very easy.
Waving hand anthelia was one of my staple LFS trade-ins.
Do like suggested, use an x-acto knife or similar to help peel it off, or to pry ever so slightly under the rock. If you can "shave" the rock it also adds the benefit of being easier to glue down the base.
But to be honest, never really had an issue of gluing them down onto frag plugs/tiles or rubble, use cyanoacrylate gel.
Put glue down onto whichever media you are using and gently press the mat/base of the anthelia down.
put it in low flow for a few days and it would have attached by then.
if any of it lifts up, just add a dab more glue. and gently resecure.
Do it a few times and you'll be a pro in no time at all!