Plain gauge spokes don’t increase the strength of the wheel. They’re just cheaper.
Spokes fail at either the hub end or the nipple end, so removing some material in the middle for lighter weight and getting butted spokes doesn’t decrease their strength (unless you go too extreme like Revolutions which are very thin), but it also increases their fatigue resistance.
Rim wise, you’ll not kill a Mavic 729. But then again unless you run 2.5″ or larger tyres all the time it’s probably complete overkill. The EX721 is a few mm narrower, about as strong, and fine with “normal” tyres as well as DH ones.
Not heard anything about the Superstar DH rim, but like anything from them, I’d be very wary…