Yes indeed the second "sli" slot can be used either for other pcie cards like scsi, network, or in fact video cards.
They do not HAVE to run in SLI. As you will not use them in SLI mode there is no need to use the "BRIDGE" connector. Your motherboard manual will tell either bios settings or placement of a switching dimm on the motherboard so that lanes are allocated between cards fairly.
The cards do not have to be the same speed or OEM vendor like powercolor, BFG, HIS.
However, I suggest that you may have less hassle if both (or all for 3+) are the same manufacturer of gpu. ie Use all ATI or all Nvidia. You may have complication trying to use one ATI card with one Nvidia card, so play safe, go all nvidia or all ati (or maybe all matrox).
This kind of setup was used for example in the demo of the Gigabyte quad royal that anandtech reviewed. The picture shows multiple (10) screens being driven by multiple graphics cards, and they are NOT functioning in SLI mode at all. If I recall, they used four PCIe graphics cards, plus an old PCI based card to give the last two monitors.
As I understand it that is precisely what you want to do, and so it can be done.
Of course there are other considerations eg winxp will handle up to 10 "virtual" screens to spread work across. In linux you should configure one per xsession and treat them mostly as individual screens. Google for how to edit your xconfig suitably.