The simple answer is NO.
You will end up with loads of problems and degraded speeds. Unsolvable BSODs will cause you to end up changing every part in the rig.
For SLI to work on older boards, it is best advised to use the exact same model and make of GPU's together.
Yes, both cards need to have the same specs or the Lower end card will be the one whose specs will be chosen as default specs of the SLI config. Means it will make no sense to have another higher card in SLI.... even Bit Wise.