Hi RohanD, this is just my opinion, but I would recommend against concurrently executing 7.5 and 9.1 on the same server at the same time. My initial concerns would be: memory on your server, the requirement to configure all the internal DevTest ports to eliminate collisions, and the strategic importance of the approach. Although concurrently running both is doable, this is not a trivial exercise. And, I would not want to set a precedence that repeats this approach each time a new version of DevTest is installed.
If your strategy is to toggle between the two, then I don't see any huge issues for you (e.g., stop all 9.1 services, start all 7.5 services. Test on 7.5. Then, Stop all 7.5 services, start all 9.1 services, test on 9.1). -- but that does not seem to be where your questions are headed.
1) LDAP: Configure Authentication Providers for ACL - DevTest Solutions - 9.1 - CA Technologies Documentation
Review this part of the docs if you are hooking into LDAP providers. You will likely need assistance from your security admins. If LDAP is already connected in 7.5, review the config as it will be helpful in developing an understanding.
2) Under 9.1, you the Enterprise Dashboard and Registry servers must be started at the minimum. However, if you are also planning to run 7.5 servers (VSE, Coord, Sims, Broker, etc) while 9.1 servers (VSE, Coord, Sims, Portal, Broker, etc.) are running you need to reconfigure all of the internal DevTest ports. For example, Registry on 2010, VSE on 2013, etc. One of the versions needs a different internal port configuration applied to prevent collisions. If you are installing and running the Demo servers, you must do the same here as well.
3) Ports: Default Port Numbers - DevTest Solutions - 9.1 - CA Technologies Documentation