We have been using the existing Webwork libraries with both Nicholson's Linear Algebra and Anton's Elementary Linear Algebra. There is plenty of material available for a first-year course.
The one warning I have is to make sure that the conventions/notation/definitions used by the questions are the same as those used by your text. We have run into several circumstances where terms in Webwork are used differently than they are in the textbook (e.g. what constitutes echelon form, the definition of characteristic polynomial, the method for LU-decomposition).