First of all...congrats that your boat doesn't leak much! Second....please don't put any holes in your boat unless absolutely necessary (e.g. to access for interior repair). Hobie didn't do it, and the fact that you're sailing a 26 year old boat is a pretty good testament to their judgement. If you'll drain your hulls each time you sail and leave the plugs out (only while while not sailing!), it'll ventilate fine. But to answer some of your questions...
(1) I read that my H16 which is a 1981 is heavier. Does that mean the fiberglass is thicker permitting cutting holes in fordecks?
(2) If not installing in the foredeck then is a different size porthole required elsewhere?
A smaller one may be required in the rear deck...not sure, but that probably wouldn't help drying much. Besides, I'm hoping I've convinced you that this question is moot!
(3) I would go with the black due to better UV protection but the installation screws are nylon, wouldn't using stainless be better?
I, too, noticed that the black claims to have better UV protection, but I went with white on a white hull where I HAD TO DO AN INTERNAL REPAIR. It's held up nicely.
No matter how hard one tries, it's difficult to get a perfect ring/deck fit all the way around due to the varying contour. The ring must be well sealed with silicone, 5200, etc. Therefore the screws carry little load, do not need to be very tight, and nylon is more than adequate. Over tightening the screws will warp the ring, and the cover will not screw on properly.
There's a very good tech/DIY article on this procedure in the On The Wire achives at the beachcats.com. (But you don't need it, right, cause you're gonna use those funds on some sailing stuff you really need and spend that time sailing?)