Hi,
I have a quick question about taking a first year elective course in second year Commerce. I already have 30 first year units. In order to get to 3rd year commerce, or even graduate, will I still need to take a second year course at one point to have a total 30 second year units?

Nope. As long as you meet your degree requirements you are fine. But there's a max of 48 units of level I courses that can count towards your 4 year degree.