German Blue Rams can generally cohabitate with Guppies, given the right conditions. Both species appreciate warm, slightly acidic water conditions, which are common ground. Blue Rams are typically peaceful, but they have been known to become aggressive, particularly during the spawning season. Therefore, the best way to ensure cohabitation is to provide ample hideouts for the guppies to escape if needed.
However, keep in mind that German Blue Rams require larger aquariums than Guppies because of their territorial needs. The tank should be large enough to accommodate all fish comfortably without competition for space. In a well-managed aquarium, both species can definitely share a tank while exhibiting their full range of natural behaviors.