So I picked up a used P320 compact small frame module. However it doesn't come with a slide catch lever. I have searched high and low, and no one even stocks or lists the small frame module slide catch lever. Is it all one size fits all.

The slide catch is part of the FCU. When you remove the takedown lever and FCU from one grip module, just move it over to the new grip module and off you go.

ETA: I see I did a nice job of not answering you question. Sorry about that. Yes, the slide catch is one size fits all. However, there are two version of the slide catch. The later version is backward compatible with the original grip modules. The original slide stop will not work with the new style grip modules.

Top Gun Supply shows them in stock.
https://www.topgunsupply.com/s...atch-lever-p320.html