well that all depends really on what skills you are looking for in a pet. I've trained a few pets, some of them where crown shop pets, and so far i've noticed that the skills they get are all kind of random. I've read up on some the info posted on fansites, such as wizard101 central, and its believed that all pets have random, predetermined skills and don't always stick to their base schools when gaining them. For example, I have a snake in a basket pet that has both life and balance skills. So its kind of like playing the lottery and hope this pet has some good skills. The one thing you can do to determine if you to keep training a pet is to see if you really like the first two skills it gets by training it up to adult. You won't always find a pet with the all the skills you want but I believe that mixing two pets that have all the skills you want between them you'll eventually get one that does. I've not tried that but according to the post on wizard101 central pet skills carry over when mixing but its a roll of the dice as to which ones so you may have to try it more then once.
Hybrid pets have mixed talents from the 2 schools of the pets that created them (usually). Unless you specifically want a pet with mixed talents, or they have a universal talent like spritely then you would probably have to breed them to make them useful. In general if a pet has good talents at adult and/or ancient then it may be worth training to Epic, especially if you plan on using it to quest with.
Pets with higher pedigree tend to have more useful talents, but not always.
I know I'm not the first to say this but pets with higher pedigree do give better talents. Also, if you're going for a powerful card like forest lord, I would suggest getting a life wizard so that you can use power pips and don't have to use 9000 crowns on a life mastery amulet. As for being worth it to train it to epic, it takes a lot of work but you can get some good talents. My friend has an ice hound (ancient) that gave her 6 resist (all), feint, and 87 health. MY ice hound (epic) gives me 7 death resist, 107 health ( I have 4071 health now), +25 speed and agility, and some mana, but i LOVE it and even when I got my wyvern I kept my ice hound because the card was awesome and it gave a slightly better card than wyvern. So it all depends on if you like the pet, you have the time an motivation, and you don't mind getting a few bad talents sometimes.
I mixed one pet in both my schools. I mixed my storm mander with my life banshee. I ended up getting the storm's resist and the life banshee's spell which worked out great.
