I have used a bunch of these, fine and coarse. Stick with the coarse thread, you will be happier. easier to thread and for some reason when you pull them up the coarse thread seems to do a better job, probably quicker on the pull i would surmise. I would use 10 size which uses a 19/64 bit, 1/4 is getting pretty big but if you want i could send you one of each to purview and i don't know your panel size, etc. the button head screws are nice, but they really suck for stripping the allen head portion, not very much meat there. i switched all my button heads to stainless allen head for the adel clamps.
i use air guns on mine, and they will mash the flange portion pretty flat, right into the aluminum. the worst thing you can do is not pull them up tight the first time, but if you mess one up just drill it out like a rivet and do it again.
if you can find a local aircraft surplus parts store get their washers, they are the AN washers i believe and don't look like the farm stuff and they are
zinc chromated, like the rivnuts.
here is where i get my stuff
https://www.lefthanderchassis.com/v2...dcategory=1145
the air guns have really gone up in price, but if you can swing one that would be my suggestion. i use around 80 psi on mine, pull the nut up then back it off and hit it again. you can lube the threads with silicone if they are not drawing up well and spray lube the inside of the gun every once in a while, just like anything else.
then when you are done you can go back and replace all the self tapping screws that were used.......