My Q60S has tanabe springs. I love the height in the front but, the rear is lower on the driver side.
can I put coilovers just on the back?

 

You can but the suspension will be mismatched, spring rates will more than likely be quite a bit firmer than your Tanabes (which are some of the lowest spring rates for aftermarket drop springs). I'd just use the full set of coilovers, you can still get the height that you like in front and will have more adjustment available for damping and overall height.