Just make them Feral Mineral Warriors and call it a night.
It's only a 2 LA and totally worth it.
How about feral lesser Fey'ri instead?
LA +1
Your choice of four fey'ri abilities; I'd go with DR 10/magic, fire resistance 10 (so your marines can be constantly on fire), and +2 bonuses against electricity and poison. The stat mods are a little inappropriate (+2 dex, +2 int, -2 con), but Alternate Form (any humanoid) allows for some adaptive armor shenanigans.
Or go for broke with feral mineral warrior lesser fey'ri.
I wouldn't suggest that degree of cheese normally, but space marines are an extraordinary circumstance.