Thanks everyone. I will check on sizing for the spa pump too.

In my case, the spa pump is on a separate circuit that is completely independent of the filtration circuit. In other words, the spa has two drain holes for filtration (which I normally don't use - only for draining the spa), two returns for filtration (which I do use, causing a waterfall into the pool), two drain holes for the spa circuit, and six spa jet returns. There is also an air blower in the spa circuit.

So in my case, I guess the spa pump sizing is primarily a function of how hard I want the jets to feel? The spa circuit is all 1.5" pipe.

If my pump curves (UltraFlow pre-94) don't show results for the low speed of a two-speed pump, does that imply that it won't work (or at least not well), or just that two-speed motors weren't common back then (1991)?