The main advantage of polygonal rifling is that the gas seals better behind the bullet, creating muzzle velocities that are slightly higher and more consistent than traditional rifling.
It also tends to be easier to clean since there are no "corners" in the bore for fouling to hide in.
But it's fine to shoot lead bullets in a polygonal barrel as long as you clean it often enough that lead doesn't build up.
The problem is that it's not possible to come up with a general rule of thumb for how often is often enough to clean.
One experiment found that two apparently identical Glock barrels, shooting exactly the same ammunition showed very different effects. One showed a minimal pressure increase after 300 rounds, the other showed twice the effect after only 75 rounds. Double the effect from 4 times fewer rounds.
The guy who did the test (Mark Passamaneck, a forensic engineer) decided that the risk wasn't worth it and switched to shooting plated bullets in his Glocks.
So yes, if you thoroughly clean it often enough (whatever that turns out to be for your individual gun & load--and there is no safe general rule to determine how often is often enough), keep away from max loads, and NEVER, EVER shoot jacketed rounds through a bore until you've completely removed lead fouling, then you can shoot your lead reloads in the Glock.
If you change anything in your load, you would need to reestablish a cleaning routine based on how the change affects the leading. Again, with no general rule to determine how to establish how often to clean.