Control Panel:
Uprights?
Technically yes, although I believe that Galaxian had a wood underpanel like Pac-Man did, while Galaga was just bare metal like Ms. Pac-Man. If so, then you CAN swap between the two, but you'd have to move the inside clamps up or down about an inch to compensate. If they are both bare metal, then they should swap easily, although with different wired connectors.
Cocktails?
No. Different shape and size control panels.
Parts : Monitors are the same, but wiring, boards, etc are different. You will not be able to plug a Galaxian board into a Galaga (or vice versa) without either changing the entire wiring, or getting some sort of adapter...