You could, if you used the legacy style. I don’t think I would try mixing the two. What advantage does your publisher see in moving to F27? The only major difference between that and 26 is the SMuFL, as far as I can tell.

Have you asked tech support for their take on it?

Thank you Mike,

I am the publisher, and I am very happy with Finale 26. I didn’t see much for me to upgrade to F27. But when possible we try to follow the technology because big upgrades often have bigger challenges. But maybe skipping this one will be the way to go...

At least the legacy style makes it possible in any case...

Stick with 26, for now. At least, until they get their act together on the SMuFL fonts. Still, I would contact them about your custom fonts.

I would have you stick with F 26 for production but get F 27 to practice on so, when the change becomes necessary, you can transition as smoothly as possible.

No reason to move to SMuFL unless you want or need to. For the most part, the SMuFL Finale Maestro etc are the same as old Maestro. If you needed a lot of built-in glyphs like quarter tone accidentals, etc, that might be different. Or if you want to use a open-source or commercial SMuFL font like Petaluma or Sebastian, then go SMuFL. 

But does that mean “sharing” isn’t a hugely compelling rationale for upgrading, along with the new icons in the Toolbar? (sarcasm). 

Of course, in the long-term, you may want to create SMuFL versions of your custom fonts. 

https://www.scoringnotes.com/tutorials/how-to-make-a-smufl-font/