Finch boxes, Robin boxes

I noticed a Greenfinch carrying dry oak leaves in through a finch-holed nesting box this morning. The box is about nine feet off the ground, framed by ivy on the wall. A Coal Tit was also exploring a Robin entranced nesting box about the same height and about three feet away from the other one. I was thinking of placing the finch-holed entrance on the Robin box if Coal Tits may be interested instead and I also thought maybe if a Robin moved in too near to the Greenfinch there might be territorial problems. I can move the Robin box further away as I have another box to put up which can have an open front, which is preferred by robins. But would a Coal Tit prefer a box so high up? I thought they preferred to be lower to the ground but we have cats in the neighbourhood so I placed the boxes high up.