Yeah, the maxes listed in the Hunters Guide are the max that a creature will spawn with.

Any animal you tame will be able to get to GM in any skill that they spawn with and all animals spawn with Wrestling, Tactics, Anatomy, and Magic Resistance.

When ML came out, alot of people were training up their Ferrets and Squirrels to 4xGM and said that they did quite well...

As with all pets, try to find one with the best resitances that you can find and feed and then stable for a week-10 days to get the bonding process over with before you take it out to train it!