Both the Neo8 and an array of the small Hi-Vi B1S will have a roll-off in their response in the 15-20 kHz area. The data that I've seen on the Neo 8 shows 5-6 dB roll off over this range. Keep in mind that the Neo 8 is more an extended mid-range driver while the Neo 3 is a true tweeter.

Assuming that the small HI-Vi drivers are closed back, I don't understand nor see any advantage why you want to used them open baffle. the driver will have no or little rear radiation. Of course you can cross them lower than most typical tweeters as you point out. A bunch of them will have enough movement capability to reasonably low for your application but remember that you are talking about very small cones with not much area moving air.

Jim