Stockman and Stovall are definitely legitimate contenders, but all seven have the sure chance of drawing votes, away from Cornyn. Holding him under 51% of the primary vote total means a run off election between him (Cornyn) and the one challenger who who attained the highest number of votes among all the challengers.
The people decide who is legitimate, by reading and listening and researching—becoming informed, and showing their support with their political action, their time and money, if they can.
I just saw some poll numbers for Texas registered voters. Sample is very questionable and I don’t think captures Tea Party voters at all and is not reflective of turnout. Kind of Low Information Voters:
Top 3 vote getters: Cornyn, Stockman and Vega. And Cornyn isn’t close to 50%.