Spindle Control 6i25+7i76 and GS3-22P0 VFD RS-485 compatible?

Yes it is, there are several posts on the forum discusing in great detail how to do that and what type of serial to rs485 adapter to use. That also depends on the type of VFD, but i can not assist any further as i have never used them.
You made a wise decision to get a 5V encoder, easier to cope with.
Regards,
Tom

Lcvette wrote: So going back to my original other question of VFD connection via the RS-485 connection. Is this something that is possible?

I would imagine so, but there isn't an existing dedicated module for it, so you would have to configure one of the generic ones.
linuxcnc.org/docs/2.7/html/drivers/mb2hal.html