OK, here's the post-mortem.
First off, it's an amazing place to visit. The number of bears is astonishing - my wife decided to count the visible bears at an unremarkable moment and came up with 10. If you're in northern Minnesota, I strongly recommend a visit, and go from opening until they kick you out. Do take some time to just watch the bears in addition to photographing.
I ended up not even bringing the Tak -- in prior tests it was just too fiddly for my patience level on my K200D. (My Spottie is at Eric's for CLA, and I'm excited to see how it is on that.) So, I used my 18-250 and was happy with the results, though next time I'll do some things differently.
Regarding FL, 90%+ of the keeper bear pics were at 250mm, and often I wanted more. The remaining 10% were all over 70mm; I think I would have been happy with a 70-100mm minimum. I think an appropriate maximum is, the longest you can afford,
as all the bears have different faces. (One bear cub was apparently wall-eyed.)
The zoom was nice, as the bears were at all sorts of different differences. I do not think I would have been happy fiddling with changing primes.
I did take some context shots at wide-angle, but they weren't essential and I wouldn't have missed them much.
The center is open evenings and that's when I was shooting. I struggled with shutter speed and almost everything was taken nearly wide-open (f/6.3) at ISO 1600. So fast lens would be nice and I should have taken NaClH2O's advice on the monopod.
Here's a few samples. The first three are aggressive crops at 250mm and the last is at 18mm.