You can mix drive models in a server just fine. You can even mix them in a single raid set.

The 2950 will probably have either a PERC5i or PERC6i. These adapters will work just fine with these drives.

Servers and PC's don't necessarily use the same drives.  SAS drives are strictly for servers and cannot be plugged into a desktop PC (without a controller card to go into the desktop).  SATA drives can be used in either servers or desktops, but there are drives designed for one or the other:

Enterprise-class drives should be used in servers, as they are designed for 24x7 use, are programmed to be able to answer commands/requests from many server-class controllers, and have the high-speed timing that a server-class controller would require.

Desktop-class drives are not designed to be used in servers.  They are generally cheaper, because they lack what Enterprise-class drives have (above).

Additionally, if the drives you have are Dell-co-branded, then you can expect to use them without problems.  If they are generic/Retail Hitachi/Seagate drives, you might expect to see some problems during the life of the drives.  Dell-branded drives (regardless of who actually makes them for Dell - Hitachi, Seagate, etc.) are loaded with special firmware to ensure they will work seamlessly with Dell-branded controllers.