I recently sold my MCX’s but kept the geissele MCX triggers I had in them. Will these triggers work in AR15s either with or without the hammer blocking thingy?

I've never tried it but if I remember correctly when the MCX first came our the triggers were so similar to the AR people were putting lighter AR but SiG discouraged it and said they would break.
Personally, I'd try it and see if it would fit and function.

By the way, why did you sell your MCX's?

I’d think it would work. As has been said, regular Geissele AR triggers fit and worked (at least for a while) in the MCX. The issue with normal AR triggers is that the MCX had a higher bolt speed and the hammer slamming back down on firing would eventually break parts of the trigger. The “fix” on the Geissele MCX trigger was to put a metal bridge across the rear of the trigger so the hammer would beat on that instead of the trigger parts. If the bridge doesn’t prevent the hammer from going back far enough to reset, and the shape of the hammer face is compatible with the AR carrier/firing pin, it should work.

That said, you might be better off trying the sell the trigger, because the MCX trigger is more expensive than a standard SSA trigger. You could probably get enough to buy a new SSA, or two Larue MBT-2S triggers.