What slope do you suggest for tweeter and woofer?
Above what freq. does a 6 1/2" woofer starts to beam?
Usually 24dB/oct slope on ribbons. The mfg should provide guidance on how to cross it, maybe on the driver specsheet?
Your SB 6.5" woofer has an effective diameter of 5", measured at center of surround. The rule of thumb is the cone diameter should be equal or smaller than the wavelength at crossover. So by that measure you should keep your crossover below 2700Hz with any tweeter. But ribbons have very wide horizontal dispersion, so you want to keep that cross as low as possible so the woofer stays wide as possible to match the ribbon off axis.
There are speaker design experts in The Lab Circle who could help with your general design questions like this.
FYI, Rick offers professional crossover design service for a fee. You have to ship the built speakers to him, but I can attest on the great job he does, and how inexpensive it is considering the result and quality of parts.
I would also recommend that you check out the "bible" of amateur speaker design, "The Loudspeaker Design Cookbook" by Vance Dickason. Make sure you get the latest 7th edition.