Secondly, I'm enrolling in units for my Law/Commerce degree and I understand you don't have to do all the core commerce units in first year. Currently I plan on doing three of the core ones, and leaving management and marketing for later years, but what can I do in the place of one of those units? A commerce elective? Something from another faculty? Or am I making this up and in reality need to do a core unit?
You can pretty much do whatever unit you like, given that you satisfy the prerequisites. Just make sure you know how many open electives you have so you don't end up not having enough units left to complete your major(s). Lots of people choose to do all core units in first year because they don't know what they want to major in or what other units they want to take.
I also chose to save two core units for later years, and instead started taking units towards my majors in semester two of my first year, e.g. I did Principles of Accounting and Finance in semester one, which allowed me to do Corporate Finance (for my finance major) and Introduction to Financial Accounting (for my accounting major) in semester two.
The standard course plans usually recommend completing your core units in first year, but you don't have to. I don't know about Law, but Commerce is pretty flexible regarding when you do your units, as long as you eventually complete all your core units and a major.