Question about the Zaishen Menagerie

ok say if i have a level 20 tiger and go and give it to the person there in the menagerie , will it possible to re charm them as level 20's there?

Yes. If you turn in a level 20 pet of Hearty or Dire evolution, you will have unlocked that pet at all levels and all evolutions.
http://wiki.guildwars.com/wiki/Zaish...unds#Pet_tiers

Like Darcy said, Hearty and Dire pets will unlock pets all all levels (not really...just the base levels of 5, 12, 15, and 20) at all evolutions. However, if it is just an Elder pet, you only get the evolutions up to 15 and you can only get Elder lvl 20s.