Bellanca

Commercial pilot privileges more-so than knowing 121 and 135 (unless you know your DPE asks about that stuff). But if you know just the basics about what 121, 135, etc., is then you will be able to answer scenario questions by knowing what operations require an operating certificate.

Other stuff students seem to have problems with (In my experience):

Airworthiness (be able to prove your aircraft is airworthy), INOP equipment (whats required, what to do if something is inop, what you have to do to fly with inop equipment), MELs, Special flight permits.

Aerodynamics - theories of lift (bernouli and Newtons), adverse yaw, effects of CG placement.

Private pilot stuff people seem forget: Airspace, Required inspections, medical duration, currency requirements (BFR, landings to carry passengers, etc)... and anything else you may have forgotten since that check ride .

Bellanca

You could do your multi-commercial in the 310 first (I personally think the multi ride is an easier ride - less Mickey Mouse manuevers). Then do the add-on in the warrior. Lots of people do that. But I personally think that's a lot for one day. I did my initial in a seminole when I was pretty much ready to pass both the single in the multi, and flew maybe 3 hours to refresh the chandelles, lazy 8's, 8's on pylons, and power off 180s then did the single add-on a week after the multi ride.

In the archer and arrow I always start my turn towards the runway as soon as I pull power back... I don't even attempt to do a full pattern. Those planes drop like rocks. Also, I like to do them at a little faster speed than best glide, especially if its windy. It seems like those planes you can always slow down and lose altitude if you want... But you can't make airspeed or altitude if you get low or slow.