About number pad: I just want to have ability to place it either on left or right side. Number pad wires will go underneath(wires shouldn't be visible) and connect to main part of keyboard. If I will do matrix, to connect then I would use some sort of connector with enough inputs. If 2 teensy's, hub is the way to go (and simpler). Number pad will be connected to main keyboard, so no second cable. So I wouldn't call it independently...
If I use 2 teensys, I could use 5x16 and 5x4 matrix's and no worries. If one, kbfirmware.com recommends 5x20. If I want to add light later on, that's not good. To save pins, I can make columns of 10 switches, but I will be pain to wire it correctly.