[1.2.323] (2) Moar Skill Balancing

So this will happen from your description of poison mist?

Also, now that DoT no longer relies on hp, can a monster be affected by the
same DoT skill from different players?
Ex. A monster being affected by three poison mists from three different f/p mages.

That is exactly what I'm saying. The example I can provide for this is how if an F/P in GMS with, say, Lv. 15 Mist (so it doensn't fully reduce a mob HP to 1) lays an infinite number of mist clouds down around a poisonable mob, that mob will continously become poisoned so long as they're within the clouds, but will only take poison damage as a status effect one "poison event" at a time. Once one ends, the next begins very shortly afterwards as the clouds are still there. Rinse, and repeat.

Also, that's an excellent question about mutli-status effects. I think if the game was able to ID whose effects were whose it could differentiate and allow multiple DoTs. Whatever the method, so long as multiple DoTs are allowed things will be fine.