One advantage with the TS1 would be greater dispersion of high frequencies. The 8-inch Fostex sounds great, but even with the whizzer, it is a bit beamy leaving a small sweetspot.
Minimal crossover - really minimal. Use a single cap on the tweeter to cut it off at 10KHz or better. No need to add anything to the Fostex. This will have no effect on the Fostex, and no crossover in critical range.
What is critical is that you get the phase alignment correct. One thing you could do (and Im sure there will be even better suggestions) would be to place the tweeter on top with the voice coil approximately aligned with the voice coil of the Fostex. Listen. Slide the tweeter back and forth and listen, stop when it sounds its best. You would not lose any benefit of a full-range driver, as long as you get the phase alignment correct.