If I run creatures x without a level list mod installed will I still get to see all the creatures ?

Probably, but it is highly recommended you merge the Leveled Lists before playing.
Wrye Mash can do it for you in 10 seconds.

I run it without and have no problems . Everything seems to turn up at the appropriate times etc , lots of ingredients adding in to general barrels that appear with nom and Morrowind Crafting goodies . Even run the Wilderness mod with it haven't had any issues .

Then you're either getting very lucky in that they're working as intended, or you don't realize that you're missing out on things. It is important to make a Merged Leveled list for the same reason that a Merged Object esp is important, they reduce mod conflicts. Merged Leveled Lists optimize the effects of the various mods acting on leveled lists and reduces an unbalanced game. Otherwise, you're just letting your load order dictate certain aspects of what will appear in your leveled lists.

edit: Wrye Mash creates the best functioning Merged Leveled List, with its Mashed_List.esp

LOL ! Probably just lucky then ! I notice a rather good thread on Level List mechanics has been bumped :http://www.gamesas.com/index.php?/topic/523132-levelled-list-tutorial/
Might provide some further insight . Cheers Pluto !

From PirateLord's readme

"Recommended that you create/recreate a Merged Levelled Lists esp. There are programs out there that do this (Such as TESTool and Wyre Mash).
Important! Do not include Creatures if you are creating a Merged Objects file. For some reason, the scripts go really screwy"

Thought I'd throw that in. Just in case.