SLI is not company dependent
It depends on your GPU. In your case, the 8800GTS.
If you have the same chip onboard on both cards, you can use it in SLI.
Not sure. You are supposed to have the same cards usually.
You can however do one thing which will make it work. Flash the BIOS of both cards to the same, specifically nVidias standard one. Then it should work.