Can voice translation rules be configured on both MGCP & H.323 gateways? Or is there some protocol-based restriction?
As above, dial-peers won't work on an MGCP gateway as all call intelligence resides on the callmanager in this setup. If you're MGCP gateway is configured for SRST then yes you'll dial-peers.
Nope. MGCP uses centralized dial-plan which all resides in CCM.
You do not use Dial-peers with MGCP