I have a 7800 SLI setup. As you said, the drivers do not allow multiple monitors when in SLI mode; there's no way around it. Disabling SLI works fine; there shouldn't be any problems caused by it. I've been running with 2-3 monitors for more than a year with SLI disabled

To be honest, I was really disappointed in SLI overall. The performance boost was not huge, even in games really designed for it (Source engine). In fact, especially in WoW, I would notice a slight but ugly 'ripple' in the screen, caused by the way SLI interlaces the lines. It's better to buy a single nice graphics card, and use the money you save to buy a better one in a year or two.