Am using bactroban (mupirocin) for skin rashes from which some fluid comes after a cut on toe. Was wondering if I put neosporin powder over bactroban (mupirocin) will it harm?

1) there is no . Point in doing that. 2) I would recommend you see a doctor. It sounds like you are self doctoring. The reason is it sounds like you have an infection and then you say a rash. So there may be two problems. If your not getting better with the bactroban (mupirocin) you may need an oral antibiotic. .
Treat and Watch. There is a possibility that the skin is inflamed and not infected. If you have been treating the toe this way and have seen little to no improvement, then maybe it is time for a change. Non-infected rashes might respond better to cortisone based cream. It is possible to alternate applications and see if it starts to clear up. Watch for increasing severity, in which case, have it looked at.