Does one need to have SAS IML installed to use the SAS/R interface? or should/could one use the sas x command to run R and feed data to it?
If you want to actually use the SAS/R interface, then yes, you must license and have SAS/IML installed as it is specifically a feature of SAS/IML (which makes sense, as SAS/IML is SAS's matrix programming language, and R is a matrix programming language).
However, you're welcome to use R the way you describe (by submitting R programs via xcmd); you will, however, need to use a CSV file or similar to exchange data between the two programs. There are several ways to do it, so look at the different options available to see what's easiest for you.
If you're choosing between the different ways to do this, here is a list of the advantages of using IML which serves as a nice comparison between the two (perhaps a biased one (Rick is the lead developer of SAS/IML), but it is sufficiently detailed in what you won't have available to you running it as a separate program that it should be helpful in making the decision).