Question

Is starting Gold 1 the highest you can possibly go?

 

To have a higher chance of getting into high division you can play duo with someone with gold MMR or higher. After first 10 games you can't get higher than plat II.

When I was playing on my brother (who has never played ranked games before) after 2 won ranked I was playing against gold III players what was a little bit strange for me

 

I'm pretty sure Gold 1 is highest, because I've been elo boosted and 10 - 0 gives Silver 2 - Gold 1, somewhere along the lines of those divisions.