Hi Razzy
Variation is always good. As my original mentor said, if you are making your practice time (be it 20 minutes are 2 hours) a job rather than fun then stop.
I did, if I didnt feel like practicing I wouldnt.. force yourself and you practice bad.
Anyway to your questions.
Yes alternate practice is very good as you learn to navigate the board better, I myself practice bull & finishes only. Being from Australia most of us have a similar way of practicing (unless have a practice partner)
You may have heard Simon on the TV say what he does for practice, same as when he was at come... so we practice, finishes, combination finishes and very important, bullseyes.
I would suggest to also practice "setting up" a finish from 200... eg: S20, bull a left finish of 130
I hope this helps but I stress the importance of practicing WHEN you want to not when you think you HAVE to as you will get disinterested in practice if you force yourself to throw