You can mix PAX and BILLY and you can mix also 2 different Billy doors (glass and wood).
In fact, I placed a MOREBO (401.040.91) (glass-aluminium) door above and a OLSBO (901.813.60) (wood) below on the same Billy shelf (400.857.09), in fact they are tall 96 + 98 cm.
The only important thing is to put the glass door above and the wood below because with MOREBO you can move the hinge where you want.
What else? Well, probably someone doesn’t know that…
The DEPTH of Billy and PAX corner are not soo much different (28 -> 35cm);
and the HEIGHT is the same if you put a Billy height extension unit (701.973.95) on the Billy shelf (400.857.09) (237 (overall) -> 236cm).. also if they really are THE SAME HEIGHT as you can see in the photo; i didn’t modify anything to have the same height!