Mandana, I walked from Le Puy on the 25th of March, 2010. While I loved the experience, the Aubrac is a brutal place when the weather turns - which it usually does at some point.
I was flattered to be declared pilgrim of the week at the big albergue in Le Puy - then they had to tell me I was the only pilgrim. Don't think I would have cracked it for the title in May.
I have to start when it is quiet and cool because I'm deadly slow and would be last in for everything once things get crowded. Also, though I live near the sub-tropical fringe, I can't handle heat. Here's the thing: if you start lower down, you should find accommodation, you'll have cold weather but nothing will be impassable, and you won't be completely alone, as there are always either pilgrims or French GR walkers about. Thanks to the GR walkers, many places are open in the cold months. I spent the night with French hikers in Saugues and Espalion, for example.
Really, any point after the Aubrac plateau will be fine. If you read the kiwinomad's blog you'll see how it's all laid out. If you could get to Conques along your way, that's a high point - to say the least!
Bon chemin
Rob