Cross over frequency should as low as possible. In Seas noedyme tweeters it is around 1.5k when used in large waveguide.
Compression driver does not fit inside voice coil.
I have done this kind of coaxial earlier based on Seas 8" woofer with 2" voice coil and it works perfectly but like to try now larger 12 or 15" woofer. Seas nedoyme tweeter of course could be used but in the large woofers the sensitiviy is usually somewhere 95 dB/W and thus it wold be easier to fit if tweeter also is more effecient.
Large woofer cone is like waveguide and it boost very much range 500-4000, so the tweeter treble range up from 4k sensitivity is what I am looking for and of course small size so that it fits inside coil.
Vifa looks promising. it is more sensitive than Seas neodymes I alreadey have 6 in use.
Cross over frequency could 1-1.5k because large cone which works as waveguide boosts that range even 10dB. Only concern is if it is too deep.