Im my area you own the meter can and the wire..the PoCo owns the meter itself and the overhead service drop to your vertical service entrance,if its a URD buried by them they own it..anything else underground is premise owned(you) Local codes or interpretations will sometimes dictate what you can and can't do.'round here they don't allow reduced neutrals..it's all up to the local inspector..you'd play hell trying to burn up a 2/0 neutral anyway..you'll be fine..just pay good attention to neutral integrity and lug torques,and the attachment to the neutral bar. Torquing AA6000 aluminum conductors-clean conductor immediately after stripping it,apply Penetrox/Nolox compound,insert into lug, tighten,wiggle conductor to settle strands of wire without breaking service equipment,then tighten more,wiggle again if you can then final torque..my critique on your panel pictured here-you have way too much romex insulation inside the panelboard..cable should be free of outer jacket insulation inside the panel(yellow #12s and black #6s)..an inspector here would gig you on that..at least you have it up and running..best of luck
