Oki, for tactics alone I'd use the basic training guide here
http://vboards.stratics.com/1894971-post11.html and simply pick the weakest creatures in the same skill group as your pet.
This is the critter list I use on little pets.
10-30 I'd go for ridgebacks and headless or better still deathwatch beetle hatchlings are passive and also around the same skill level.
From 40-50 you can hop to deathwatch beetles or try zombies
50-60 skeletons or corpsers
60-65 ratmen or trolls
65-70 Lizardmen and ogres
70-85 I usually start on the gaman around this point and stuck with them till 85
85-95 Harpies. I use the Ilshenar nest and pull the harpies out one by one so it doesn't get crazy
95-100 if you're training the pet on a tamer, then there are obvious options of sparring a tame pet vs the new one. If not, you can keep on with the harpies or use either yourself or a summon to tank while your pet trains on something else. I usually use a unicorn tank with a little pet and take them to kill rotting corpses at that point, but it just depends what you have at your disposal.
Of course it's even easier if you have a tamer's stable of pets to team up with your weaker one as the tank, or another tamer who can tank for you. But tank pets aren't essential at all, I pretty much took my pack out all together and trained them at the same time. It just takes care and good heal support
Wenchy