I remember I had some problem installing the 750ti too... (and I ended removing most of everything... so it can be detected...)
Try first to run the cdrom which came with the 750ti. If it doesn't work you might have to uninstall catalyst I am afraid...
Actually I already have a bunch of 280x running on my rig with AMD drivers (13.12 if I remember correctly). Anyways, I went forward and plugged the two GTX 750Ti ASUS. I downloaded the latest drivers at Nvidia's website and installed those. I placed the 2 750 Ti in the first two slots and the 3 280xs in the following slots.
To my surprise all seems to be working perfectly fine. I am using a fork of grs-sgminer for the 280xs and ccminer for the 2 750 Tis.
Thanks.