Hey team,
I was just planning out an army for Southern Song and I usually start with Magna as its a little more achievable from a painting PoV but with an eye to expand to maximus.
That said Imperial Infantry in Southern Song have a notable difference
Magna - UG size 4 with max of 12 bases - allow 3 UG of Infantry
Maximus - UG 6,8 with max 16 bases - meaning the max you could get is 2, either 6 or 8 strong.
The Later Northern Song can field 18 bases when fielding separate formations as well.