If i don't take the paracetamol and take only the codeine. will it still be an effective painkiller on it's own?

Possibly: Many pain meds are combined with paracetamol to help boost their effectiveness. Now there is a big push from the fda to limit the amount of paracetamol in these combination medications to reduce the chances for accidental overdose since there is paracetamol in so many other combination meds. You can safely take 3000mg of paracetamol per day.