My G25 shifter didn't work at all with the G29. It is different to the G27 shifter though, but I'd take that as a sign that it won't work with it. Especially, given that you have to purchase the shifter separately with the G29 AND G920.
G29 pedals are the same EXCEPT for the brake, it has 50-60% of pedal travel which feels normal compared with the older pedals but then the last 40% of travel is stiffened out massively with what feels like a block of rubber on the back of the spring. Many people have described the brake pedal on the G29 as being like 'xyz brake mod'. So it might be exactly the kind of brake pedal you want out of your pedals.
I got my G29 for 180£ from Currys, it wasn't my first choice of wheel, and I only needed a new set of pedals, but it didn't make sense for me to buy ANOTHER g25 or g27 used, so I just went and bought the thing.
To expand on other things, the FFB in the G29 is applied almost identically to the G27, though it feels more robust, actually, the logitech software is limited though, so at this time, you can't adjust global device settings [other than the rotation].