Are the prereqs for graduate Dentistry and Medicine at Melbourne Uni the same?
Yes
So if you do the prereqs for Medicine, will you also be able to apply for Dentistry?
Yes
Also, if you're doing the prereqs for Medicine, will there be any space left over for other subjects?
This depends - see below.
Say, if you want to also do the prereqs for engineering?
This depends a little bit on what subjects you do/don't have to do in first year, but it sounds like you'll probably be fine because you're preparing early.
There are, as you know, three second year level prerequisites for Medicine.
These come with you also needing to have taken up to Chemistry 2 (so, if you took Chem in high school: Chem 1 & Chem 2) and also both first year Biology subjects (no credit for high school Biol). I think one might even require a maths subject (Calculus 1 or 2).
Engineering requires Engineering Systems Design 2 at least at the first year level, as well as Calculus 1 (if you didn't take Spesh), Calculus 2, and Linear Algebra at first year level. Chemical Engineering Major requires Engineering Mathematics, Chemical Process Analysis 1 & 2, Transport Processes, and Reactions and Synthesis at second year level.
So already you have 8 x Level 2 subjects and 5 x Level 1 subjects to fit into your study plan, and that DOESN'T include breadth. But if you are taking Specialist Maths 3/4 and Chemistry 3/4, it's probably will be able to fit it all in.
I had to go speak to someone at the Science Student Centre and in the end I couldn't fit it all in my study plan - I was two subjects short - but that's because because I didn't take Chemistry or Specialist Maths in high school, I had to take Fundamentals of Chemistry and Calculus 1, which bulked up my study plan too much.