It seems fairly likely they could pull off a win. The entire force in endgame was like a couple hundred people, the overwhelming majority of which where just dudes with guns or the like. Bucky had an entirely conventional machinegun and more than held his ground during both battles, a Custode armed with a vastly more powerful weapon should be more than capable of replacing dozens of other soldiers. The biggest issue is the ship and Thanos big stuff like the flying whale things. A bolter, even a beefed up Custodes bolter, does not have the punch to bring those down.
The most likely outcome I can see is the Custodes kill Thanos, most of his lieutenants, and a major chunk of his army but are eventually either overwhelmed by the hordes or the ship glasses the area. With Thanos dead there's no one left to use the stones, but if the stones survive the bombing whoever's left of Thano's forces will recover them.
The less likely senario is the custodes try to use the stones, which would probably work better for them. If Tony can withstand the force of holding them all long enough to fire off one last quip, it seems likely that a physically, genetically, and mentally superior custode would be able to do the same.