We are planning in being in Castelrotto In early May 12,13,14 of 2018. In reading RS book on this area I noticed that the Cable car from Sius/Seis suggests it does not open until mid May. has anyone been there before these dates and taken this cable car, or should I alter my dates?
I think you're pushing it a bit by being there that early in the season. Here's a similar thread that appeared on this forum earlier this year:
https://community.ricksteves.com/travel-forum/italy/dolomites-d137631c-fe42-4677-9733-1311a882db6b
From one year to another weather conditions will vary, but I'd recommend delaying your visit if you can.
This year, the Siusi-Ape di Siusi cable way did not open until May 24, and there is no reason to think it will open any earlier next year.
http://www.valgardena.it/en/hiking-biking/walking-trekking/lifts/