If you would actually SLI AGP card and PCI-E cards together, first, without taking the fact of the downclock of the PCI-E card, it will also not be able to work. The AGP interface cards are using different architectures from the PCI-E, and it will cause catrastrophic crash (Ex, a big explosion followed by fire).
SLIing two different GPUs will make it not to post. You need exact GPUs on each. I don't think the brand would matter though. Correct me if I am wrong.