RE: select fins
When you buy aftermarket fins, it's fairly normal to have to "fit them" to your board.
Not sure what tools you used, but a good double cut file is really the best tool to use as you can take off material only in the places where the fin root fits tight against the fin box.
Find a bar of bath soap, and rub the fin root all over with it. It will be a whitish dull grey color on a black fin.
Then put it into the fin box and lightly snug the screw (unless the fin root stands well proud of the bottom of the board).
If the fin root sticks up above the bottom of your board, you can just insert the fin and wiggle it fore and aft slightly.
Pull the fin back out and the shiny black areas are what you need to file off.
Just a few strokes with a douoble cut file on the radius at the front and rear of the fin root and the fin should begin dropping down into the fin box.
Keep applying the soap, and placing the fin in the box until the flat surface where the fin root joins the foil is just slightly (approx. 1/32" or 0.8 mm) above the surface of the bottom of the board.
That last little bit of "sticking up" is the "draw" you want so your fin fits good and tight.
Now install the fin screws and pull the fin root down flush with the bottom of your board.
When you remove the fin, you will need to tap or strike the front of the fin to loosen the fin in the box, because of the "draw".
When you have a perfectly fitted fin, it will slide in with some effort all the way, and you will see almost full contact (shiny areas in the soap) all the way along the front and rear radius.
As far as the "cracking noises" that's not real good, and usually indicates that there were some high spots in the fin box that needed to be smoothed out as your fin was being fit to the box (use a small round or "rat tail" file for this).
Since the fin box is a carbon fiber "cassette" the cracks near the screw holes should not be a problem if you use a nice large dia. washer and a good seal washer that's as large or a little larger than the washer.
The cracks were caused by you over tightening the screws in an attempt to make the fin root fit into an imperfect (or slightly too small in some dimension) fin box.
Hope this helps,