can you rezone multifamily/commercial to singlefamily?

In most cases, yes. Certain localities are reluctant to "downzone" properties because the potential for tax revenue is typically higher based on the commercial zoning. Depending on the property, it could make more sense to develop it as a single-family. I would contact the zoning administrator in the locality the property is in and discuss with them. You may have an argument that the particular property is not likely to be used for multi-family or commercial use and that your single-family use makes more sense.
Best of luck.

Why? You would potentially deminish the value of the property.
However to answer your question ... yes but it depends on the political climate of your city. Many cities want the higher density for tax revenue and as a means to provide affordable housing. However, many want to reduce density. It all depends.