Technically, creatures with less than 3 Intelligence are "unplayable". However, if you were to improve the Int score (say through an awaken spell), they should work.
Basically, the LA for animals would be as follows:
+1 for a fly speed (+2 for good or perfect maneuverability)
+1 for climb or burrow speed
+1 for swim speed (if in an aquatic-based campaign only)
+1 for scent
+1 for natural armor
+1 for every 5 ft. of reach over 5 ft.
-1 LA if Small
I never understood that "creatures with animal intelligence/severe public ostracism/bizarre anatomy/any other large handicap can't be playable characters" mentality. If my Sanctified Vampire, Boarder Collie, and Awakened Gelatinous Ooze players can’t pick up a sword or make an intelligence check with a DC of 5 or go anywhere without zealous villagers trying to stab them with wooden stakes that’s a thorn in their feet, not mine. I don’t mind torturing my players, in fact I jump at the opportunity.