I think you should decide in advance how powerful the guards are and stick with that. No changes to help or hurt the player characters. The Black Box has a handy chart showing the classes, levels, and gear of enforcers in the different domains. Instead of adjusting the guards to the PCs, just use what seems most fitting and plausible to you. Drakov's soldiers with military weaponry are probably a cut above some Nova Vassa bully boys armed with cudgels.
If the PCs pick a fight with town guards and win, they may attract the attention of more guards. If it gets really out if hand, a mob might form. Or, much worse, they might come to the attention of the darklord, if this is a domain in which the darklord is the ruler.
If they lose, but didn't kill or maim any guards, maybe the judge will go easy on them. Hard labor, banishment, confiscation of gear, whatever. Again, I think you should go with what makes sense in game.
If you want to tip the players off that the local regime is oppressive and brutal, describe stuff like man-cages hanging from the town walls, prominently placed public gallows with fresh bodies swinging, the fearful looks the citizens given armed watch who are just passing by, etc.
Then, if they kill some guards over a petty dispute about weapons and end up dangling from nooses later...
Well, you did give them plenty of clues.
Heck,
"NO SWORD PERMIT " or "STABBED WATCHMAN" placard hanging around the neck of a corpse on the gallows by the town gate could save your party, if they are willing to heed the warning. Do that once and they might get it.