As far as I know, it is already option on other sites to merge your account now as part of the process to integrate Curse and Twitch accounts. It's not mandatory now (but neither is the Twitch app - just encouraged) but I don't know about the future as I don't work for Curse Media. However, if you've already merged your account for the app, it should already be recognised on Curse sites and you should be able to successfully use your Twitch login there. It won't impact our post history or display name or anything like that; it's basically just a login thing. The only real difference is that on the Twitch app you kinda want to merge early if you can so you can get the name you want on the app. In the case of Curse sites, you can pick a unique display name per-site that doesn't need to match your overall account handle, so it's less critical.
MMO-C is an exception among all Curse sites just because it is using its own software right now and has it's own login system that goes with it.
But yeah, if you don't already use your Twitch account to login on other Curse sites, you'll be able to once you merge your account.
Edit: here is the FAQ for Curse Media sites and merging
https://curseweb.zendesk.com/hc/en-u...15002024687-QA