Well I have a shubunkin, and when I introduced two smaller shubunkin into its tank, it tried to eat both of them and only gave up when they wouldn't fit in its mouth.
Goldfish are pretty voracious eaters and if it fits in their mouth it is fair game. Even if they could not eat an adult, they could probably easily eat younger fish.
I have seen people keeping pairs of bettas in larger tub ponds (think around the 40-100 gallon mark) over the spring/summer and then bringing the fish back inside over winter. You are most likely not going to get huge numbers of fry by leaving both parents in, as I would think there is a very good chance either one or both parents would cannibalise them. I don't get any cannibalism by parents in my wild betta tanks, and even then, I only get a dozen or so fry growing out per tank.
This is because it's the predation from older siblings you also have to watch out for. I have had juveniles wipe out entire new spawns in a matter of minutes as newly free-swimming fry are extremely vulnerable.
So I think it's probably a slim possibility you'll end up with hundreds and hundreds of fry at the end of it.
However, I personally wouldn't put the bettas in with goldfish. I would think set-up a separate, smaller pond, and you are probably going to be much more successful.