You cannot have both legacy and non-legacy drivers installed as they have file name conflicts (and probably more). You can use a certain version (maybe 12.4, check the release notes for all until you find it) which support both cards. You won't be able to upgrade ever though.
#1: Yes, using that certain driver version.
#2: Yes, it would not only break it, but you'd get a system that BSODs on bootup.
#3: That certain version should work for both. Upgrading that is not an option however.
#4: Nope, that's the same as #2.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64