How can I disable iMessage for some of my contacts?

I have unlimited text, but limited data on my iPhone 5s. I only want to receive/send iMessages to a select few on my contacts list who have iPads and then use SMS for my iPhone friends. I tried disabling my phone number as a receive/sender, but it only lets me disable my email. How can I make it so I don't have to turn off iMessage to send SMS messages to other iPhone users? It's very problematic. I end up receiving all my messages from those with iPhones on iMessage when I turn it back on, which defeats the purpose of me sending an SMS message to them in the first place. If I disable iMessages completely it keeps me from being able to communicate with my iPad using pals. I can't win either way.