So that means you can only appoint starting from the 2nd in line?
Exactly. If you want to disqualify your firstborn, you should change to elective. Then you can make him a bishop and by that, disqualify him from being available to be voted.. Note that merely
naming your sons heirs for bishoprics is enough to disqualify them. So, in gavelkind, name your extra sons (option only available when they turn 16 - if you have only underage sons in gavelkind and get maimed etc., just give them bishoprics right away), they will get nothing when you die, and your firstborn gets all the secular titles. Then, if you have still some another use for those same sons, you can change the plans. Like marrying them to foreign countesses and duchesses.
Naming your sons to bishoprics removes also the penalty for having unlanded sons in elective and primogeniture by the way. I think it applies for the seniority, too, if the penalty is in it, too.
I rarely bother to appoint heirs to regular bishoprics (other than sons) but I pay attention to get the ablest men in prince-bishoprics and prince-archbishoprics. I pay attention for example stewardship if I aim to expand my culture in county level.