Comparability between RAZR, RAZR Maxx, Maxx HD, etc

[Snareman]

I don't quite know/understand how similar all of these phones are. Will a ROM for the RAZR also work on the RAZR Maxx, MAXX HD, etc or are all of the phones different? To me they seem like the same phone with 4 versions - 2 with regular screen, 2 with HD, 2 with small battery, 2 with larger.