Yes Bitcoin is the most gambled, and Ethereum is next.
I have a crapload of monero which I like as a coin, but it's a small % of the gambled coins. Would it be prudent to trade some monero for say litecoin or something?
Are you planning to invest it on a casino's bankroll ? or just gamble with it? Monero still has a decent number when it comes to gambling stats. On
dicesites Safedice's overall stat for monero is sitting at the top 10. Spreading your investment on different coins is a wise decision because it lessens the risk if one coin doesn't go according to plan you still have other coins to back you up.