Holster Shrinking

You may be able to mold them to the pistol, should be able to find a vid on how to do that. Best to have the pistol or a mold in it though or you may shape it into something which doesn't work for you. It will get a touch smaller but not going to shrink a lot, not sure how much you need it to go.

May be better off to sell the $$ holsters as they are, or may luck into a trade somewhere with them.

I'm sure we'll have some holster makers chime in with the real answer, but shrinking a holster would most likely be for getting your old G27 holster that may have stretched out a bit, back closer to what it was when new. The concept isn't for shrinking the holster down in size for a smaller gun.

Kydex holsters just seem like a natural for Glocks. I'd just pick up a new kydex holster or two for your G43.

Holster type cowhide leather won't shrink under any normal situation, it can only stretch.
While it's possible to re-mold a holster to fit a slightly different gun, you can't shrink one to fit a smaller gun.
The leather that shrinks is chrome tanned leather as used for boots and gloves.

If you attempt to re-mold a holster you're actually stretching the leather to fit around the details of the gun better.
Once put in use the leather opens back up and the fit is even worse.
The only way to tighten the fit of a cowhide holster is to put in another line of stitching tighter against the gun, or install a rivet closer in.