Hi, you can attach center-pinch lens cap on this filter , except the ND1000 filter , the CPL, UV and magnetic adapter ring can attach lens cap. 

Hello! If you use the magnetic UV filter separately, or use the magnetic adapter ring together with the CPL filter, you can use a normal lens hood on it. Does not require a larger thread. If you use the magnetic adapter ring together with the ND1000 filter, you cannot use another lens hood on it. 

1. The UV filter can be used alone. ND and CPL need a matching magnetic adapter ring, because the camera lens itself is not magnetic and cannot be directly attracted to ND and CPL 

I've never used Hoya ND's, but I have GOBE, KERNEL, ZOMEI, and K&F ND filters. Out of all of them, I like the K&F the most because the colors look great out of the camera and they don't create a vignette around the edges like some of my other nd's which I hate. Because of the great out of camera colors, it gives you more to work with on post-production when you shoot on a flat profile. Also, I've used them during night photography on bulb mode to do light painting and the 4 - 6 stop ones work great for this. Overall, I think for the money the K&F filters are worth it. Awesome product! 

It is a normal phenomenon if you want to use ND filters together or ND filter with other filter, like ND2+ND4,ND4+ND8,ND2+ND8 or ND2+4+8,ND filter+UV, the photo effect will become to light pink color, you could set "white balance" as "AUTO" to improve it, but will not be completely disappeared.