Wargs are exclusive to the Fallen races, so yes, you can only use horses and they can only use Wargs.
That said, unless they changed it from Beta, if you capture a Fallen city that had a Warg resource harvested, it should produce them for you 
As for memory issues, Elemental can only use up to 2gb since it's a 32-bit process. You could try googling for a "3gb enabler", which is commonly used for some 32-bit executables for games that are memory heavy. These don't modify the executable itself, they only give it a "large address aware" header so the OS can give it a little more wiggle room. It will be fully compatible with MP and all that.
