The pet I use largely depends on the deck I am playing with. The Wolf pet is great for most defensive decks, as it can get some early swings and and stay up to defend.
The Hound pet is really great in aggro decks. I use it a lot with vampires and the like because haste is just awesome.
The Bird is great for most control decks and combo decks, as looting helps you get the lands/cards you need for the situation. I think it would actually be more powerful if it tapped to loot, though.
The paths idea sounds ok, but would be less ideal for people who like making a lot of different decks, unless you allow unlocking each path as a playable pet, given enough credits.
One major idea you could go with here, I think, is making one path utility and the other aggro. Here are my suggestions: Red: a 2/1 haste hound with the ability it has now, or a 0/2 haste snake, with T/Ping for 1. Green: a 3/1 ape with trample, regenerate G, or a 0/1 squirrel with G, T/Search for a forest, put it into your hand. Black: a 1/1 bat with flying and lifelink, or a 1/1 rat with sacrifice this card, each opponent discards a card. Blue: a 2/2 seagull with flying and can only block flyers, or a 0/4 turtle with 1, T/Loot. White: 2/1 wolf with exalted and vigilance, or a 0/1 dove with flying and
X, T/Prevent the next
X damage to target player/creature.
I also question the plant a little bit, as to why it is the only one in that path. Perhaps a defender of each color, rather than just a green one?
Those are only a few suggestions, but they were what came to mind.
Explore the colors a bit, I am sure there are many animals that could be considered familiars for each of the colors.