Installing the SQL server software on your system and getting informatica to talk to that are entirely different things.
1.You can for sure install SQL server on your Informatica server system, till the time you are dealing with some varient of Windows. Informatica can be installed on *nix flavours but not SQL server. If your informatica server is not on windows varients, you need to have a separate windows box for sql server installation.
2.Getting informatica to talk to sql server is going to be dependent upon the platform keys that you have for Informatica. Having the driver and everything is fine, but finally informatica allows interaction to OS/databases based on the platform keys, these keys are normally purchased and have therefore price attached to every additional environemnt/database you want to access.