I haven't played using the Doom miniatures in the game whenever I play without expansion (I still use the Doom ones if I play a scenario that requires the expansion, or if I play against more than 1 marine), instead, if playing on an 'original only' scenario against 1 other marine, I use these and I explain why:
Marines=Space Marine (Naturally, the new terminators are too big. Had I own some old school terminators, I'd use them)
Zombies=Genestealers (Most numerous, just want to relive Space Hulk)
Imps=Termagants (Primary Ranger)
Trites=Spore Mines (Pest in 40k, Pest in Space Hulk)
Demons= Hormagaunts (40k don't have any 2-space bases, so the hormagaunt's overhanging body serves as the other space)
Mancubus= Biovore (Big Guns, 'nuff said)
Hell Knight= Warriors (What more do you want?)
Archville= Zoanthrope (Mass psychic)
Cyberdemon= Hive Tyrant (Yeaaah, the big boss)
The reason why I only use them against 1 marine is the fact that I am NOT getting 6 biovores (1 a box, so I need to buy 6 boxes, which is crazy) or 3 Hive Tyrants (I can only use 2 in 40k...). And the reason it's not expansion enabled is the fact that... tyranids don't have enough species to cover the lot.
Have a say about my choices. I don't plan to change anything, but I can do with some comments