Capacities will not be the issue here. You are not running into trouble adding the DS1, it's only when the Integrated CSU option is attempted on the Add/Change DS1 Form that experience the trouble. This is due to the following reason, which I found in the Avaya document; Installation, Upgrades, and Additions for Avaya CMC1 Media Gateways-
555-233-118
Issue 10
June 2005
"The basic ICSU requires a TN464E or TN767D or later suffix. The enhanced ICSU requires a TN464F/GP/HP or TN767E or later suffix. The enhanced ICSU can also be used with the TN2464 or TN2313.
So, it's really a matter of hardware! You need a supported vintage of circuit pack, is all...