The .38 super is, I think, a larger cartridge, than 9mm or .40, so THAT follower is unlikely to work in a P-01. The .38 super is generally associated with the large-frame Witness models.
The 9mm and .40 round are generally used with the small-framed versions and use the same mag tube (with feed lip adjustments.) The .40 follower should work if its NOT made to work for one of the conversion units and much larger mags. You may have to call to find out. I don't know if any adjustments would have to be made to make the .40 follower engage the slide stop.
A factory mag for a P-01 might work in a pre-B, but only if it is not flush-fit on the P-01. The flush-fit mags are "compact" mags and would be too short for a pre-B grip frame. The mags with the extender/filler are full-size mags; with the extender installed, they wouldn't work in the pre-B.
Pre-B guns are the hardest to find mags for and, generally, only true factory mags made for the 75B or pre-B work without problems. Your best bet is to buy some factory 10-rounders for range use, and look around for some of the pricier 15-round hi-caps if you plan to carry.
(Forum members have said that the new Mec-Gar 16 round mags don't always work in the pre-Bs. Remember: while Mec-Gar makes factory mags for CZs, a Mec-Gar made factory mag is not the same as a Mec-Gar mag for the CZ-75B.)
I'd be very surprised if any 9mm mag could be tweaked to 20 rounds. Angus has been working on a new design and has 18 rounds working, but has said that 19 rounds problematic, so far. (I get the sense that he'll eventually have the 19-round mag working, but its not there, yet.)
Angus has made no mention of 20. There's only so much space in that mag tube; more than 19 would probably take a new mag tube. You've got to keep enough space in their for springs! (Perhaps a new technology spring is the answer?)
I think that after 15 or 16, paying a lot extra for a round or two more seems a waste of money for anything except the big IPSC gun games. There, in IPSC, it may be very cost effective, and offer financial rewards.
For range of carry, the extra two rounds aren't going to be as useful as spare mags.