I can't really give you any advice on transferring to UoM and prerequisites, that's just something you'll have to look into. There's plenty of info online. If you're still confused, talk to someone at the science office.
In terms of your Monash units, I would suggest completing first year sequences before you commit to a major. You seem a bit undecided on what your interests are. There's nothing wrong with that, just don't dive straight in without getting your feet wet first.
As for first year chemistry, year 12 chem is not a prerequisite but it is required knowledge. I'd definitely suggest CHM1031 as you have no prior experience. You can transition into the main stream in semester two (CHM1022) if you do well.
So my advice is to keep the maths and biology sequences, and chemistry too if you're interested. FIT1029 isn't an especially difficult unit, the amount of content is actually quite bare. The university introduced it to help first years learn how to approach problems and create solutions, instead of just rote-learning answers to classic problems. It's a good introductory unit especially if you're doing any programming, but by no means necessary unless you're completing a CS minor or major.
If you're still undecided on where to place the extra 12 credits, I'd recommend another first year sequence just to expand your horizons. If you find yourself interested in pursuing a minor or major in one of these subjects then you'll need those first year units under your belt.