I am not fully switching from trumpet though. I need the mouthpiece that is the closest in size to a cornet mouthpiece. I know all about ruining embouchure and stuff. All i need to know is what mouthpiece is closest to a trumpet mouthpiece that could also fit in a trombone or baritone

no replies! I shall be patient and wait it out!

I'm not worried about what damage you are going to do to your embouchure (cuz if you don't care, why should I?)...
I'm more worried about how you are going to sound with a mouthpiece that small.
The smallest mpc I'd say go for is a 12C. Your tone will sound like a laser beam if you're not careful though.

I played cornet for a number of years until I moved to a bigger instrument and realised I'd been playing the wrong instrument half my life LOL.

Mouthpieces are very much an individual thing, but I will say that on balance, a large mouthpiece is better in the long run, particularly if you practice regularly. You are more likely to get a better sound long term. I've just progressed to a Dennis Wick 4AY this week and I believe it is better for range and endurance (providing the practice the right exercises).
Don't worry about your embrochure. Providing you practice regularly and avoid going back to other instruments/mouthpieces, your embrochure will adjust within a few weeks. Good luck,

Don't worry about your embrochure. Providing you practice regularly and avoid going back to other instruments/mouthpieces, your embrochure will adjust within a few weeks

yeah....still don't believe that...