On the Aubrac it gets close to 1400m elevation, it's a high plateau. Think of the scottish highlands, maybe. In april snow is not unlikely there. In may it can happen also but it's not as common as in april.
May is a very popular month for the french people to walk shorter sections, especially Le Puy-Conques, so it will probably be busy. Beautiful though during that time of year, with all the spring flowers, and the temperature not too hot. You'll probably have rain and mud as well as warm and sunny days.
I walked it solo in april-may 2017 and really liked it. Had no snow, but a lot of mud.
Now, walking it in summer at the moment, it is just as beautiful - less crowded, but the heat is extreme.
Happy planning. Good choice of Camino