can you change iMessage settings by contacts on iPad Air? I would like to be able to get texts on my iPad running iOS 8.1.3 from some people (work contacts) and not others (Wife). Is this possible? or is it an accept all or none feature?
Actually yes it is possible, although it isn't as automatic/intuitive as would be nice. 

On iOS 8, it is possible to mute any message thread. What I recommend that you do is go through and mute the conversations that you do not want coming into your iPad. You will no longer receive those notifications. 

To do this, simply open the conversation, tap Details, and turn Do Not Disturb on. 

Perhaps, in a future version of iOS, we will be able to control these things on a per-contact-group basis.