what does 5 quick race and 10 quick race mean?

It means that you have to win 5 quick races or 10 quick races as fast as you can. You also HAVE to win all races.

i just realised since our current account are in high leagues (bronze, silver etc), we get put with better people. so are we allowed to make new accounts to speedrun with the aim of being put against worse players?

If you want to. I am using my silver account but you can create a new account if you want to.