If you go for a shadow ele, I think you'll need another pet tanking against the ele. Otherwise I fear the swampie might get swamped
(one day I'll resist a bad joke!) Also you need to get the swampie to the same skill as the shadow ele to get the best gains.
I would start your swampie fighting gaman to work melee skills, as they aren't too squishy or too tough. They're easy to get to and they're passive gentle creatures so you won't be overwhelmed with them or their damage. I would try those for as long as you can before looking for new spawns to work with.
The trick to training any pet in UO is to match it to a spawn with similar skill levels.
The Hunter's Guide here on Stratics has a very useful advanced search for pet training. Lore your pet, look at the lowest skills first. Go to the advanced search in the Hunter's Guide and down at the bottom you can search by skill levels. Put the lowest skill in there and search - you can set different criteria for sorting the results too. Then it's just common sense because you choose spawns that a swampie could handle i.e., the easy ones. It's best to start easy and go to tougher spawns, rather than go hunting something too powerful and getting your swampie killed The HG will even tell you where to find the spawns you want to try.
If you can, include any small magic casters you can too, then you'll work up magic resist as well as melee skill.
Wenchy