I dont think feeding mussies only rodents has any ill effects, i had them for a couple of years & they did great on mice, very solid muscular snakes & beautiful glossy irridesent sheen. Really cool snakes.

Up against an Indigo size for size mussie would probably win out because of the venom, having said that it depends who grabs who first. The feeding response of drymarchon is incredible & the speed they can take down prey is awesome so if an indigo or cribo latched onto a mussies head i think it would be bye bye mussie.
Even with constriction, i have seen Dry`s eating constrictors & they just keep swallowing it down.

Luck of the draw on the day i guess.

The only other snakes i have kept that have a feeding response like Dry`s are False water cobras. They are awesome too.