Were Melta Weapons used during the heresy??

Meltaguns were. I think I read that Inferno pistols were old, too (from old fluff on Dante's weapon), so they're probably ok. Melta-bombs are similarly probably fine. I think that multi-meltas were restricted to vehicles only, though (i.e. no Devastators).

But all plasma related weapons are alright?

No, they're the same as Melta's.
Pistols, -Guns, and probably any form of grenade are alright for squads.
But Cannons and ordnance size versions were vehicle mounted.

Right on, the reason I asked was I wanted to include them for my Dark Angels army but since they weren't used yet that does save me some time. Thanks for the heads up!!