There's definitely advantages to shooting with established brands for pros... But you can't shoot with what you can't bring to the field, period. (And you can't make money on what's too expensive to recoup) By the time a few years go by, Pentax might be caught up in some of the respects like AF speed, and lacking a model with a blistering FPS rate, a full metal case, and whatnot, and in the meantime, there's lenses that'll still be worth something.
As JG says, there's more to it than just the specs, ...can you get parts, service, do you end up working for someone and not being able to tap any equipment pool or rent or borrow odd things that are more available... Are you familiar with and quick on said stuff...
And it's not easy for a camera company to break into that kind of market and compete on those terms.
Still, for you right now, I think the important part is to get you making images... all that stuff doesn't matter if you aren't getting the shot, thus making money. You do what you have to.
Wildlife stuff is easier to go it alone with, mind you, if you think Pentax has a body that'll stand up to the duty and you can get the kind of lenses you'll want: they're pretty serious about putting what you need for good images *in your hands,* which is very good.
It's great for me, cause I'm not likely to leap up and become an AP stringer with my health as it is, but I still take it seriously... (I like to think maybe I can break even with this enterprise, and that's something that's a lot more possible with Pentax. If I start making big bucks and need more of certain kinds of performance, then the money's less of a problem, anyway, so taking any resale hit on Pentax stuff wouldn't matter, come down to it. Invest in your skills, as a first priority, then check out the numbers.
Probably best to plan a kit out for each, and add up what it really costs, for what you need.
But, you're already in Nikon...