[Sasha]

You can mix and match all components except the electronics - battery, motor/servo controllers, motors. For that, you need to choose either Tetrix or Matrix - no mixing allowed.
I would suggest Tetrix as the main platform (battery/controllers, axles, gears,...) with AndyMark Neverest motors. For structural pieces (beams, channels...) you can use tetrix, t-slot aluminum profile, or servocity Actobotics set - take your choice. We are using 20mm t-slot aluminum, plus a lot of just aluminum profile from HomeDepot whcih we cut and drill as needed.