I like the effect of reborn worms whenever they are sliced or chopped, as worms can do this indeed.
Unfortunately this is not true. However, if it's verisimilitude you are after, a lot of worms are simultaneous hermaphrodites and reproduce really fast, so merging into slimy sperm pods before splitting into four could totally be on the table. *gross*
Porcupines should be made "giant" and get AC/HP/damage all increased to fit better with their depth.
Personally I think the lair has enough tanky loners. I agree that a mirror damage (maybe proportional? though this might encourage self-nerfing with trash weapons - or just give it a cap to remedy?) would be interesting. The wiki has the damage returned from 'spiny' as HDd4 (a wimpy 2d4 for porcupines) - can anyone confirm this? Apparently this mechanic is being used for the new Hell Sentinels, so i suppose that porcupines would just have to scale up to make it more effective. However, alone it runs the risk of just encouraging kiting/punishing melee without offering any interesting alternative. Perhaps they could be given a big speed upgrade along with higher HD and a weak to average melee attack making their spines their main source of damage output. Spawn in groups of 2-3? This would move them out of pure popcorn into the zones of annoyance/harass monsters. This could be accompanied by a change in flavour eg. they roll into an offensive spiky ball (giant hedgehogs???) upon noticing the player with accompanying large buff in speed and small buff in damage, in this state they cannot travel on stairs. This leaves an armoured melee player the options of putting up with annoyance/harass damage till they can get out of the level or just duking it out and taking the corresponding larger hit in returned damage, and a caster the always prevalent option to nuke, or to try to run early to escape the high speed.
RatsPutting mechanical considerations aside for a moment, I would desperately love to see the flavour reworked for these guys. Green, orange and grey are hardly imaginative or interesting handles. "Plague" for green and "gnarled" for grey seem like two possible alternatives that took a whole two seconds to think up. Orange rats are perhaps the most frustrating because of the lack of indication that they have draining attacks (I've always assumed that they must be a shout out to something, otherwise the mind boggles).
Mechanic suggestion: replace grey rat with "rabid rat" for an early nerfed version of a moth of wrath. Spawns with other rat packs, attacks nearest thing that is not enraged with a weak melee attack for a chance to make it berserk. (could also potentially berserk the player?). Not sure if this would be very effective, but it might serve as an interesting introduction to a mechanic that can be potentially overwhelming for newer players, plus also would increase drastically in danger depending on what appeared in the area.
Off topic/auxiliary suggestions:
-escaped white mice swarms in wizlabs with chaos brand attacks.
-Why if there are so many rats is there no pied piper styled unique? (Eustachio doesn't count!) Summon rats + smite-targeted/within-earshot resistable make player dance ability. Make it happen people!
+1 to all the suggestions about tracking hounds. That would be really fun if it could be pulled off.