Yes, you can add filters in this holder, two to be exact and as far as the thickness of the filter goes, I have no idea. I own several different name brands of filters which I use in this holder without any problem. All the filters I own fit nicely into the holder and for myself, I have never really given any thought to the filter thickness, however you will be able to fit only one filter per slot. Sorry I cannot be of more help but this filter works great for my photographic needs. 

No. This filter holder can only hold 2 filters. I believe the more prudent course would be to get a filter that is rated for more stops and can replace two filters that are rated for less stops. There are filter holders that do hold 3 filters but they are smaller than the K&F filter holder (which is 100 x 100 mm). And in my opinion, the quality of those 3 fitter holders that are located on Amazon is less than the K&F system. 

your lens curvature will require an adapter for any filter system. I saw the Lee filter adapter for this lens...but it seems it does not fit all filter holders. Lee filters will work with the adapter. 

Hi，this is a square lens filter kit , has 8 adaper rings can fit the lens size from49mm-82mm, and the filter is square , 100*150mm/100*100, if you filter size is same , it would be ok ,thanks. 

Hello, this is a high end filter kit. Of course these filters are made of optical glass. Regards. 

one filter 

It came with the 10 stop glass filter and seems to be pretty great quality. I'm loving using it. Even ordered a 6 stop to go with it. 

No, you need to thread the holder onto the lens itself. No, you need to thread the holder onto the lens itself.