I have a customer that wants to add more digital extensions to their current DSX 80 all they have is the CPU/Power/VM Cards, a 8 port CO card, and 1 16 port ESIU card. Anyone foresee any issues with me just adding another ESIU card to the cabinet? Iam just not sure how many ports the cabinet can take, and if the cards need to be in specific order - i read that at least one of the ESIU cards have to be in first port after the main board.
Any thoughts?

You can have 2 digital 16 port cards.
After that 1st one cards can go in any slot.