Realize that if having higher-level veterans, assassins, and gladiators for the party to face is going to make them unable to suspend disbelief... the problem is having 9th and 10th level PCs, not the idea of higher-level NPCs.
If you want a city with all normal-level guards in the 1st, 2nd, 3rd level range and have them be credible threats... then you can't have PCs that go higher than like 5th or 6th level. If you keep all your PCs in E6... then you can continue to use suspendibly-disbelieving commoners, adepts and guards all you want and have them still be able to take down and arrest the party should they get uppity.
But if you let the PCs get to 9th and 10th level and higher... then you either have to cede "reality" and get guards to match, or you accept the curb-stompings and let the PCs turn into psychopathic villains.
If you want a city with all normal-level guards in the 1st, 2nd, 3rd level range and have them be credible threats... then you can't have PCs that go higher than like 5th or 6th level. If you keep all your PCs in E6... then you can continue to use suspendibly-disbelieving commoners, adepts and guards all you want and have them still be able to take down and arrest the party should they get uppity.
But if you let the PCs get to 9th and 10th level and higher... then you either have to cede "reality" and get guards to match, or you accept the curb-stompings and let the PCs turn into psychopathic villains.
