lt says in the rules only one Environment Mythos card can be in play at all times. As there are different types of Environment cards i.e. Urban & Weather, are they allowed to be in play together or is it only one Environment in total allowed.

Only one total. However, there are some Ancient Ones that blocks a certain type from being played.

DoomTurtle said:

Specifically, the old environment isn't actually discarded until you go to apply the new one's special text. In other words, if you have an environment out that makes fast monsters move like normal monsters, and you draw a new environment mythos, fast monsters will still move like normal monsters before the environment is discarded and replaced (monster movement occurs before mythos text).

Also, with Ancient Ones like Ithaqua (weather cards are discarded without their effects taking place), you actually do everything else on the mythos card first: open gate, spawn monster, move monsters, clues... instead of applying the environment text, the card is discarded.