Sorry if this sounds like a stupid question.. but why do you have to do the practice exams in a particular order?
Edit: Oops, didn't realise this was in the Biology board, so sorry that my answer isn't biology specific and only based off the VCE subjects I've already done.There's so many of them, it makes it easier to sort out which one your going to start off with.
Also, some companies have better quality exams, or more difficult exams, or more easier exams etc. so you'd probably want to decide if you want to do harder ones first or mix it up and have a variety etc. There are heaps of different reasoning to different orders, i.e. if you can master the more difficult exams first, then the VCAA ones will probably feel like a piece of cake etc. Or you could do the easy ones first to ensure you know the basics well, and then do the difficult ones closer to the exam so that you can work on weaknesses better etc.
Also, VCAA is obviously the most important, so it's up to you if you want to do those first (so you're familiar with what's expected) or if you want to do them last (so you know where you stand for the exam, last minute improvements based on exactly VCAA's style of questions etc.)
Personally, I prefer to do VCAA first, random mix of other practice exams and then VCAA again in the last couple of weeks.