Can I merge different MODIS collections?

For the snow cover and sea ice products, users can use MODIS Collections 6 and 6.1 together. Users should be aware that there were minor changes in calibration for C6.1 that could potentially induce some differences, however the impacts of these changes on the snow and sea ice products should not be significant.

There weren't any changes in the science algorithm, so the two collections are largely compatible.
Although archived, 'Golden Month’ Collection 4 data as well as Collection 5 in its entirety are available upon special request. Due to differences in science quality of the products, users are advised not to use these collections in conjunction with newer MODIS collections.