Tony,
I am assuming that we're talking about using the caps in parallel, yes? You can do so in series, but it requires care in execution, and I don't recommend it.
By all means, as long as the voltage rating for each cap is sufficient for the rail, you'll be fine. I once took a whole box of caps of varying capacitances and voltages and wired them up to one of my old Thresholds. The total was something over a quarter of a Farad. Very solid bass, indeed. It worked well, although it looked like the aftermath of an explosion in a parts factory. Speaking of which, one of the caps did eventually bite the dust, but it was age, not the voltage, that did it in.
Grey