Re: any reason to remove excess old fiberglass while grinding your hull?
You want clean, smooth glass to promote adhesion. Grind away enough so that the new glass will not "bridge" low spots and the surface is completely clean. Gelcoat on the surface can cover up problems underneath it. I would recommend removing enough of the surface pigment so that you can verify that there is no voids or dark spots in the laminate. If you lay glass over what is already there without checking you risk laying new glass over bad glass.
Delaminated, or poorly laid glass, will show up as a white areas in the glass. Glass that has been contaminated or possibly holding moisture will show as dark discolored areas.
My boat had allot of these bad spots in it. I was grinding for days. But at least now I know the new glass will stick and I am not worried about trapping water in the hull. (causes freeze cracking) Fiberglass is not impervious to water.
Make sure you are satisfied that you are laying new glass on a clean, undamaged, uncontaminated surface. Even epoxy wont stick to dirt. And of course you never want to lay glass on top of paint.