It is the controller that is the limit, not the rack…
So, no matter what rack you use, controllers with serial numbers less than 600,000, ONLY the first 8 slots can have a mix of digital or analog.
In short, no, you cant put your eight allowed 4 channel modules just anywhere on the rack, they must be constrained to the first 8 positions.
Your guess is correct, no matter the rack, the connector on the bottom of those older controllers only has digital inputs for the first 8 positions.