Fear Face FX
How to attach horns to any latex mask without a socket for the horns. [production mix and match type of deal]
This depends a bit on the size, shape, and weight of the horns. Smaller and lighter horns can be glued on, and you can even paint a bit of latex around the base to help seal/blend them in. A glue like E6000 should do the trick.
Heavier horns may require more support, and for that, you can either have a small plate under the mask they attach to, a headband-like plate for both horns, or a full plate over the top of the head on the inside. I would use a couple of small bolts through the plate to lock into each horn (you can set a couple of threaded inserts into each horn) so it can't twist, then hold the plate in place with some adhesive like E6000 if you want it to be more of a permanent addition.
The plate can be thick vacuum-formed plastic, fiberglass, thermoplastic, etc.
If the horns don't match the mask well, as you mentioned it's a mix and match situation, you may be able to use latex and cotton or tissue paper to make a paper mache sculpting material to add small details around the horn. It can be pigmented by mixing in acrylic paints. Then paint/finish as desired.
Hope that helps!
Fear Face FX
Thank you so much Chris, you were a tremendous help!!!