I would say that The G-30S will not fit the G-19, G-23, and others. Some older Leather Holsters work fine, because they have stretched. Most of the full Kydex, holsters will not work.
Just something to think about. When I bought my first G-30S, all that I heard was that they were the same size. When I got it home, surprise, I had to pull out my box of holsters to find one, that it fit. Luckily I had a couple that it worked fine with.
Due to my size and shape, I do not get along with inside the waistband holsters. I let my Bladeteck holster go with the Ruger LC9s I let a clipdraw spring go with it also.