I'm sure this is an obvious answer, but only to those who have physically seen the switches in question.
So, you buy/design/steal a PCB. It was built with the expectation of, probably, cherry MX keyswitches. Matias makes alps clones which some say are nice. Some users swear by Topre mechanical.
Since you can't tell by staring at the keycap, what actuation is going on underneath, are these switches interchangeable? Can I desolder ALPs keys from an ebay find and put them on my phantom, or ergodox?
If not, do we all, as a community, assume that PCBs floating around all expect Cherry MX unless expressly stated otherwise?
I guess a second question, related, is "are there MX clones" although matias didn't start, I'm guessing, until ALPS quit. But still ... can I arbitrarily get "generic" mechanical switches that will work with an arbitrary PCB I might be holding in my hand?