The Pentax users will have to make a big step up from the current level of postings to justify having their own sub-forum.
Sony, Olympus, Fujifilm sub-forums were only created when there was enough postings in the Others category and where the members themselves petitioned for a sub-forum.
So, the first step to having your own sub-forum is to prove that there is a ground-swell and enough members/posts to sustain the momentum (because just having your own forum is not enough, must also make sure that there is sustained postings and activity, otherwise, the sub-forum will be pruned when the perception is that the forum is under-utilised).