I thought I'd chime in here too Ruppy, as you specifically mention PPLAUS2015 - I can speak to applying for AUS - and as is stated above, each Region varies slightly.
Generally, when you submit your Gym Leader Application, you nominate a Type or Theme - but you're not locked in stone to that particular Type. The "Locking In" happens later, once the Gym Leader forums have opened up, and you [i8ir]formally[/i8ir] submit your Leader thread. If this makes sense? eg, Leaders swap type after being accepted to ensure we maximize our Gym diversity. As we aim for PPL to be maximum FUN, diversity is important.
In the case of Frank Underwood, not only did Kaka have a great theme idea AND a solid mono-Water Gym, he also had a fiery desire to take on the Champ position
and ultimately he edged out a returning Leader for the spot in a 5 match mega-gauntlet of Fainting and RNG.
AUS may do things a little differently next year, but if you want to be the best, my suggestion would be to keep asking questions
and obviously getting involved on the forums!