Rue has, what appears to be, a hole in his tiny little Betta head. At first, I thought it was a scale missing. But the scales on his head aren't that big. It just looks like a little white spot on his head. What should I do about it? Could it spread to the goldies that he's with? Should I isolate him with some meds?

What should I do?

Just to be safe, isolate him and make sure he has a heater on at around 80f to keep his immune system up.
Any chance of a pic? as that would really help id what he has.
Do you have salt?

No salt. I can't get close enough to take a pic of him.

My betta had the same type thing you're describing, his kinda looked like a pimple on the side of his head, and I salted his tank and did water changes and it disappeared. I have no clue as to what it was but the salt seemed to really help. He perked up and the place got smaller and smaller and finally went away. I have a bristlenose in his 10 gallon with him and he didn't catch it but I would separate him from your goldies just as a precaution better safe than sorry and plus as someone else mentioned a warmer temp will help the little guy too! I hope he gets better ! Keep us posted! Here's a fuzzy kinda bad picture of what it looked like:

Hole in the Head is caused by a naturally occuring bacteria in goldfishes' stomachs and generally only affects fat-bodied types of goldfish and a few other types of fish.

It's meant to be fatal but if you catch it early and dose the fish with some anti-bacterial medication, it doesn't have to be. I saved a little goldy by giving it a 3 day bath in double strength Pimafix.

What's on the betta is probably another kind of bacterial infection or a little ulcer caused by ammonia build up or bacteria. Bettas really don't do well with goldies. Goldies are little guzzle and poop machines where as bettas are much more conservative in their output. Maybe because goldies are largely vegetarian and bettas mainly carniverous. Dogs don't poop as much as cows. Goldies are the cows of the fish world. Basically... you put a dog in a cow shed, and he doesn't like the greenstuff on the floor. And he'll probably find the cows a bit pushy and the cowshed a bit drafty as well.

I suggest you move him to his own tank for at least a week and change the water every second day. Add any anti-bacterial product you have, or if you have aquarium salt, up the dose of that slightly, or both. Bettas really don't like other types of salt. Make sure the water is dechlorinated and keep his tank warm with a heater or lamp.

The clean water may be all he needs to recover if the hole isn't serious and he's healthy.

If it does have a pimply look, than its bacterial. Hole-in-head is a rare disease most common in cichlids, rare in goldfish, and very very rare in bettas. If its hole-in-head, it will look like a litle dent. It is generally thought to be caused by long term lack of light, nutriets, good water quality etc. It used to be believed it was a parasite, but fish scientists are coming off that idea recently. I say go with a bit of anti-bacterial meds like yabbie said, because it is most likely bacterial.

I have him in a 1gal by himself with a tiny bit of Melafix and a tiny bit of salt. I've been keeping a close eye on him and so far all is well.