No, you don't need to order Rhythm of Handwriting for Foundations students, since Foundations already teaches handwriting. The same rhythmic, stroke-based instruction from our Rhythm of Handwriting curriculum is built into the Foundations lessons.
Rhythm of Handwriting is designed to be used alongside Essentials (which does not teach handwriting) or as a stand-alone writing program.
Several of our handwriting resources are used in both Foundations and Rhythm of Handwriting: Tactile Cards, Rhythm of Handwriting Quick References, and Student Whiteboards.