Well, the KEF centre won't balance up too well with the Missions - in fact it'll stick out like a sore thumb - but of course it will work well with the KEF left and right speakers when you get them
What would you recommend? at the minute i wont be able to buy the KEF Q7's for 6 months and the current centre is very poor voices sound tinny and quiet.
Well, you can either put up with it until you can afford to buy all three speakers, or if you are certain you're going to get the KEFs even if something better comes along I'd buy the KEF centre now and play with the set-up on the Onkyo to balance it up with the Missions.