This article over at WoW wiki details battle pet experience gains! I think you'll find what you'll need.
How catching affects level:
- Catching any pet level 15 or under retains its current level.
- Catching a level 16-20 pet will lose 1 level.
- Catching a level 21-25 pet will lose 2 levels.
- The XP required to level for pets after level 10 has been dramatically reduced.
If your pet team prevails in combat, experience is awarded to all
surviving pets that participated in combat. Pets that do not enter
combat have no effect on the experience awarded. Similarly, pets that
die in combat also have no affect on the experience awarded. The
experience that dead pets would have received is not "lost". Instead,
the system considers dead pets the same way as non-participating pets.
When you defeat an opposing pet, the experience awarded is calculated
using this formula:
[opponent level + 9] X [level difference + 5]
The level difference is capped at +2, so there is no benefit to
defeating an opponent more than 2 levels higher than your pet. If your
pet is 5 or more levels higher than the opponent, it will receive zero
When your opponent is level 5 or higher, there will be two of them,
and you receive experience for each of them (200% normal experience).
When your opponent is level 15 or higher, there will be three of them,
and you receive full experience for the first two, and half experience
for the third (250% normal experience).
The quality of the defeated pet does not affect experience gain, and
you get full experience if you capture or defeat your opponent.
The experience awarded to each surviving pet is calculated by dividing
the experience in the table below by the number of surviving pets:
For more information on battle pet experience, see the WoW wiki article.