Yeh, you're definately getting pointed in the right direction of a betta for that tank. =)
And about the whole betta-living-with-a-goldfish thing. Normally, in bigger tanks, bettas prefer a heater while goldfish are strictly coldwater fish. While a betta can be fine without a heater, I find that the female bettas I have in a heated tank are a bit happier with warmer water.
Just from personal experiance.
I've often thought about seeing how well one of my male bettas would fair in my goldfish tank... but I have yet try such a thing.
Good luck with your fish purchase! And I'm so very happy to see someone doing research before buying the actual fish. Good job! =D