I hope this mod gets finished if I've always wanted to work on a mod like this, I can't mod though so this mod makes my dreams come true.
Just wanted to add.
For the crawlers you could replace them with the chimera.
They are very similar to the crawlers (both are bug like, and crawl around), I've seen their models from RE1 Remake (gamecube) used in Garry's mod before so there's a working model out there some where.
For the scrakes you could always use DR salvador from RE4,
For the stalkers you could use the Novistador from RE4,
they look similar to the crawlers (kf) or Chimera (re1) LOL so nothign like a KF stalker, however they cloak them selves and go invisible just like the stalkers do in KF so they could be work?
For the flesh pound if you stick with the RE1 Tyrant I think it would be cool if you had his heart change colour when he rages, Like how the Flesh pounds chest changes colour when he rages in game.
If your thinking of changing the RE1 Tyrant to something else you could try the Tyrant T-0400TP from Re Outbreak series.
I have seen quite a few models that have been ripped from the Outbreak series used in Garry's mod so you should be able to get hold of the Tyrant T-0400tp Model. He looks a little bit more menacing than the RE1 Tyrant.
For the Patriarch Birkin seems a good fit as they are both scientists who are horribly mutated and have created monsters that attack the player. However Nemesis seems a better choice due to his use of the rocket launcher. But I suppose either could work quite well.
If you stick with Nemesis however I would like to suggest maybe changing his model a little bit to a Semi mutated version. As he has tenticles coming from his body which would work well with the Patriarchs chest snake (wtf it is) attack. (i'll try and find a picture for this one)
For the the Gorefast you could use the Crimson Head zombies from RE1 Remake.