I have moved to cheaper android phone. I still have my old iphone6 and a newer iPad. If deregister by changing messaging and FaceTime on my iPhone can I still use those features on my iPad? I

Good question.

Typically iPhone users have two ways to send/receive iMessages: using an iPhone telephone number or an Apple ID.

As you will no longer be an iPhone user, you should de-register that telephone number. You will still be able to use your Apple ID to send/receive iMessages on your iPad.

This should be well explained up in the article linked above.