you should be able to use any MX-compatible switches, you can't use optical, magnetic, alps or choc.
the brand can make a difference in key fell but any should work in the sockets, although desoldered switches can damage the socket, so no vintage switches is preferable.
the pcb is the part connecting the switches electrically, the plate does the same mechanically, the pcb mount switches have 2 extra plastic legs for support when you do not use a plate, as you said, you have a PC plate, so it does not matter.
the gat ink blacks should be compatible, and switch testers are nice but lack a lot of features to fully try a switch, like a feedback for actuation and the feel over multiple fingers and row, so maybe do not spend hundreds on it, and if you are pretty confident on what you'd like maybe order a 10 pack of those instead and put them in the board.
hope this helps
