Q.What varieties of pampas grass are suitable to zone 5b in southern Maine?
Female pampas grass.

Certified GKH Gardening Expert
Erianthus ravennae is considered hardy pamapas grass (to zone 4) but its plumes aren't as thick as the pampass grass hardy to zone 7.
The dwarf pampas grass is hardier, Cortaderia selloana ‘Pumila,' hardy to zone 6 and semi-hardy to zone 5.
Most propagated pampass is female; males don't have as full and silky plumes.
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm
https://www.gardeningknowhow.com/ornamental/foliage/ravenna-grass/ravenna-grass-information.htm