I have access to REALLY soft water and REALLY hard water, So I can mix them to pretty much any level. What level is best for goldies?

I dunno. If i had to pick between hard beds and soft beds id pick soft but im no expert.

I'd make GH around 100ppm and KH around 120ppm

My KH is 300+ way off the test strip. Is this bad? I've never been able to lower it, and the only thing I've heard of to bring it down is muraic acid. But that was recommended for large ponds and I'm afraid I'll kill my fish ._.

I don't think it's a problem. You could mix in some distilled water to bring it down. that should lower both GH and KH.

What is distilled water?

It's bottled water that has no minerals in it.

Would it be different than Kenetico drinking water? Thats what we have for drinking water, and then the whole house except the hose is very softened water.

Yes. Check the label. They usually say what they've done to it. I think drinking water is usually fairly normal water with minerals in it.