I know that if you have a bonus on a main skill, you get that bonus automaticaly on a special skill.

eg: +1 fight gives also +1 combat because combat skill is based on the fight skill

But what if you can reroll a skill check, does the same thing still apply?

eg: if you can reroll a fight check, can you reroll a combat check?

Yes, everything that refers to a skill check also affects the special skill check related to that skill - bonuses, rerolls, whatever.

-Villain

+1 to Villain.

Combat checks are a subset of Fight checks. Everything that applies to Fight can be applied to Combat (but not vice versa!)