20mm is the “right” size, but they might be listed as 20.1mm or some rot like that. They fit.
The inside of the front bezel keeps the TIR more or less flat, and as long as the LED is centered in the hole, all should be okay. XM vs XP (5050 vs 3535) shouldn’t make much difference, especially with a wider-angle lens.
Ideally, TIRs should have the “shoe” that acts as a centering ring and spacer, lifting the lens just a smidge off the pcb, but it’s not critical. I wedged one in there, had a hard time doing so, but it worked. Also worked well without it, either.
Just get an assortment, play around with it, and you’ll likely find one that suits you.
60° is a nice flooder, especially for a headlamp, but also in a S2/S2+ that’s mainly to be used close-in and you’d want it to be nice and floody.