Is it permissible to say "peace be upon whoever follows guidance" when greeting a Muslim?

It is not permissible for a Muslim to greet another Muslim by saying: "Peace be upon whoever follows guidance," because this is the phrasing the Messenger of Allah, peace and blessings be upon him, used for non-Muslims. As for a Muslim, one should greet him by saying "Peace be upon you (plural)" (السلام عليكم). By saying "Peace be upon whoever follows guidance," the implication is that your brother is not among those who follow guidance. If there are Muslims and Christians together, one should greet them with the usual greeting of peace, intending it for the Muslims.