Are group messages sent over iMessage when possible? When I select multiple recipients, both iPhones and other phones, they are all marked with the green color, as if the message will be sent as an SMS.

Will the message be sent over iMessage to those who can receive it?
From what I have seen the minute a non iMessage user is added to a bulk message it sends all the messages via SMS. I suspect however this is dependant on the carrier more then apple. 

From testing, the verification of whether a user has iMessage enabled is done before the message is actually sent. 

My best advice is for you to try it out and see. I don't think there is really anyway to force it use iMessage.