The Bullock Precedent only applies to redistricting. I think you'd have a lot of angry Republicans if the Senate tried to apply it to setting primary dates.
Actually, if you read the editorial written by Dewhurst (posted on another thread here), you will see that the 2/3 rule has been suspended at least 20 times in the past 50 years, and only a few of those times were for redistricting. Also, Perry could make redistricting and changing the primary date part of the same special session. If there is no blocker bill in place, each piece of legislation is addressed in the order it emerges from committee. In that case, the Dems would need a 2/3 majority to KEEP the bill from being debated, by voting to take up another piece of legislation instead.