Not in our case. We use shapeshift.io for all Altcoins and don't need a Altcoin bankroll. All incoming Altcoins are immediately converted to bitcoin and the other way around if one likes to withdraw Altcoins.
I heard the size of the bankroll doesn't matter, and that it's safe to offer 20 BTC payouts no matter what kind of bankroll you have. Because PrimeDice do it, and so DaDice can do it too, "for commercial reasons".
You should allow gambling in altcoins without converting to BTC. If anyone wins, pay out as much as you can and declare bankruptcy if they bug you about the rest. This would be the same as your stated plan for BTC bets if anyone wins too much. Simples.