As for tactics, I'm still an utter newb with them though I've been playing DoomRL for a couple years now.
I ought to clarify that one a bit. It used to be multiple different tactics in an old release but nowadays it's a button for an evasive mode (the game notes the tactic as 'running' but it's not running proper, I don't think). Increased chance to dodge shots (though that won't help against an Arch-vile or Revenant, it will help somewhat against Hell Knights, Barons, and just about anything else with a projectile attack), movement speed increased (I think the wiki says 30%, although I don't know if that's an older release or the current one), but a slight accuracy hit.
After you use it, you need to either heal or go to the next floor to use it again.
Plus I went with chain fire, and I never thought of trying to cancel it (I imagine there's still a cool down while you wait for the barrel to stop spinning and firing) in order to pop a couple medpacks.
If there is a cooldown proper, I haven't noticed it much. I think any 'cooldown' is built into the firing time of a weapon.
Plus cooldown didn't really apply much to chainguns in the original Doom.
By the way, did Halls of Carnage ever show up as a possible special level for you? It's one of four levels I know of that has a BFG9000, which, in addition to coming in handy against enemy hordes assisted by Arch-viles, is really recommended in most modes (I can think of maybe two challenge modes where it's not needed, and in both cases you couldn't use one in those modes anyway) for the endgame.