Thanks for the info, guys. The reason for using 8 is ... it's more than 4? 
No, seriously, I don't know how slowly I might want to run this thing. My 1st ball mill, ya know. From what I've seen on the web, running faster is not very likely, and it varies depending on the grinding media and what you're grinding. So I'm just planning for high versatility. I do like the idea of alternating poles, simply because it seems a bit more robust. If I use a fairly weak glue, shouldn't be any trouble to flip the poles on 1/2 of them, but based on what Grumpy_Mike said, I'll try running them the same way first, though I guess that with a unipolar hall switch, I could run them alternating pole, and it wouldn't matter, then I could switch to a bi-polar if needed.
@jackrae: The 2 reasons I can think of for not using a reed switch are avoiding debouncing it, and just that I'm a bit leary of mechanical actuation in this case, even though, as you mention, they can be highly reliable.
ETA: The advantage of the reed switch is I can drive to Radio Shack, or maybe even home depot and buy one. Might still try that, since I really don't have any big reason to make another electronics order right now, but I also have this irrational :~ objection to software debounce, and I don't think I have any Schmitt triggers laying about.