In theory, bts/bitasset pairs would be fine, but in bitshares there tends to never be nearly as much liquidity for the bitassets versus external assets like BTC and ETH. For this reason, I think it would be better to stick with ETH for now. But using pairs from multiple gateways is a good idea assuming there are multiple reliable gateways as this would prevent disruption in the case that one of the gateways has it's deposits/withdrawals temporarily disabled.