Here is the rules section that "I" believe covers that topic.
But what if your beloved tribe only has seven
different cards? This way a legal deck can never
be made — and we were so close! Only two cards
short of the minimum...But never fear, a legend
may be recruited in those special occasions. Just
think of a legend that could conceivably be of
your tribe.
So let’s say seven different hippos existed and a
twisted mind would want to build a hippo tribe
deck. He would have to ask permission to use a
legend as a hippo. When he approaches me to ask if
Rith, the Awakener is a viable hippo, I’d slap him
in response. Or maybe I’d tell some one to slap
him. (Or maybe I’d just tell him I wouldn't allow
it.) But would he ask me permission to use
Phelddagrif, I’d pat him on the back and wish him
luck with his deck.
Cavemans liberial interptation:
I'm very open-minded about using cards that make a deck
fun to play. We use Legends in our decks all the time, even
if we have enough "other" cards to compile a decklist. But
the Legend(S) in question have to relate to the Tribe in some
way. In my mind Mageta and Jareth could be cat tribal cards.
If someone wants to use a card that might seem to be
questionable all I ask is that you have a logical reason
why it belongs in your tribe and you are willing to support
it with a response.