Just to clarify: SMS/MMS need cellular data, iMessages can work without cellular data (wifi) or with cellular data (if there's not a wifi network available).
That said, in Settings>Messages you should have "iMessages" ON, and also "Send as SMS" should be ON.
I might have misunderstood your question, but I don't see any problem.
iMessages automatically go to the recipients who have an iPhone with iMessages enabled.
Shouldn't be this case, your message is sent as SMS.
Regards
Giulio