I've found that while using binoviewers I've never wanted to dim the image with a moon filter. Because it splits the light between both eyes it effectively already works as a 50% decrease in brightness per eye but you don't notice it because you're using both eyes and your brain is amazing
So, I wouldn't bother about a moon filter at all personally.
The binoviewer itself doesn't increase the telescope focal length; ie. your telescope has a 500mm focal length whether you use a diagonal, binoviewer or straight through.
The 1.7 and 2.5 GPC do work as 1.7x and 2.5x barlows though and you'll likely have to use at least the 1.7 for the binoviewer to come into focus.