Not sure what you mean by putting everything in session 1, the max you can do is 4 units/subjects a session IIRC
Personally I do 4 units and the workload is very reasonable, at least based off of first year. I also worked part time for majority of it about 16-20 hours a week,it was doable and I think I got good results as well.
Some students may prefer to do 3 units, particularly later on when the subject matter gets harder, but I think you should try 4 in the first sem to see how you go.
People and planet units are basically electives that you choose from a set list. I think our degree only has one elective anyway, and regardless you shouldn't use it in first year (Imo its the same with P&P units, but most people I know wasted them already, but I'm saving them for later on as I can balance out harder units with an easy first year unit this way.)
I can't find it right now but there was a recommended list I just used in my first sem units, but either way our course/degree is really structured and you can pick whatever mix.
