Where can I find a list of internal airline designations for various aircraft/cabin configurations?
Internal designations are usually used to differentiate various seating configurations or differences internally for an airline. From your example, all those codes are various codes for the 757-200 (at Delta, they code the 757-300 as a 75Y). You can often look up various seat configurations at Seatguru.com, a site that generally tracks seating information on airlines. Additionally, all would be sent to ATC using the ICAO type B752.

The reason for these variations can be broken down to a couple areas of differences. One is exit configuration; in the same type (757-200), there can be 2 or more different configurations of emergency exits (combinations of doors and windows, and various types). If you have a fleet that mixes these, you'll often have to have different seating configurations for them. Another differentiation could be whether there is a subfleet equipped and used for ETOPS flights. There can be other reasons an airline could want a subfleet and a different cabin. Also, if you're in the process of upgrading a fleet, you might want to create codes to differentiate them so you know which is going where and be able to plan your revenue models around that.