The 5.6 release notes state:
"Upgrades from SME Server 5.0/5.1.x/5.5 are auto-detected. Upgrades from e-smith 4.1.2 are also auto-detected. Upgrades from previous e-smith versions are not supported."
Based on this information, you should be able to go from 5.1 to 5.6 directly, and bypass 5.5
Hope this helped,
Nathan