will different faceplates work on other main units.
ex alpine 7873 faceplate on a 7878 or vis/versa

I suppose there is the chance they might, but i would generally say no. Each HU has different options and features. So the main part to the HU is gonna have trouble transmitting these features and options ot the faceplate if it doesn't even use them

Yup, Alpine and Panasonic have model to year compatabilities with their faceplates.

Are Matsushtia/Panasonic and Alpine the same company? or have some close ties etc?