I have a 22, so not completely familiar with the 25, and also not sure what the aesthetic considerations are, but with that said:
1) I tend not run screws into "just" fiberglass, because it's not generally all that thick, and also shortish screws into fiberglass don't grip all that well. (Although, as Barry notes above, great strength is not needed, so a variety of ways can work.) If it's cored there, then while you would probably be fine running into core from the inside... I still tend to do something else.
2) I'd likely use the Weld Mount wire tie bases (with the slight caveat that if they are going to be in plain sight they are a bit utilitarian looking). These are small (or large), white (or black) plastic "bridges" that glue onto the boat (with strong methacrylate glue intended for the purpose) and then accept a wire tie. I've used these on my 22 to hold wiring and tubing for the trim tabs, cables in the battery compartment, etc.
You can find them at
www.weldmountsystem.com. Here is a photo (and oh, great, this caused me to see that they now have a new, even stronger, aluminum tie mount :wink
I also like to use the low profile clasp cable ties (most times). I use Cobra brand - was introduced to them as mouthpiece ties on SCUBA gear. They don't have the square "hump" at the clasp and it's even easier to cut them smooth at the clasp (although not impossible with regular ones). There are a few times I prefer the standard type though.