Quote from Amuraivel
1. How many cards per tribe would be necessary to make a tribal draft viable?
The more the better. Tribal decks are linear, so they get weaker for each card less available to them.
2. How many tribal cards would dilute the power too much?
This q can't be answered because 'too much' is a matter of opinion. The answer could be 1 and correct or 20 and correct for another person.
2. Which creatures/cards would be best for each tribe?
Lords, and cards that gets better the more creatures of the teibe you control. And for zombies 'gy matters cards'.