If you can find an Onix before a Geodude, I envy you. Can you perfectly avoid Zubats too? :P But yes, that's true.
You're right and wrong on the Nidoran evolution lines though. Since they're only ground types once they're evolved to King/Queen, you need to find a moonstone. And of course, nobody in the world has avoided Geodudes long enough to find that first moonstone.
Although you are correct in that they're the first pokemon eligible to catch for your monotype challenge. I shoulda thought of that. >_>;