You can likely just copy/paste the saved variables from the live version of some of the ported addons for non-classic specific things (like bar replacement addons positioning and what not).
As others said, you can test them during a stress test. The next one is June 19th. If you haven’t already, make sure you signed up for classic beta testing on the beta test opt in page.
you can get some classic addons from https://www.wowinterface.com/downloads/index.php?cid=160&sb=dec_date&so=desc&pt=f
This thread talks about addons being ported: https://us.forums.blizzard.com/en/wow/t/were-porting-retail-add-ons-to-beta-come-get-em/
In the addon porting thread an addon developer talked about that and said they were working on it.
If I open the twitch app right now and set it to classic beta it actually says the addons I downloaded from wowinterface are up to date and doesn’t try to update them to a retail version so it might already be working? The get more addons section still shows all the retail addons though.