If I'm not mistaken, FX owns all their feeder aircraft (Caravans and ATRs) and contracts those other companies to operate them. For that reason, FX maintains much more operational control over their feeder system than UPS, who just contracts with the lowest bidder. I think Corporate Air is based in Casper, WY, or at least has a pilot base there. I don't know where Baron's HQ is, but they fly around Nebraska quite a bit. Wiggins is based in the northeast, either VT or NH.
I have a friend who flew Caravans and as F-27 First Officer for one of those companies.
By the way, he left his pilot career about two years ago, by choice. And he is from another country. I can ask him to read PPRuNe and offer you some information. 'His' company often did not use seniority to upgrade pilots. They hired some outside guys to put into some Captain seats. First year Captain pay was much cheaper.
It was cheaper for the company and he became so disgusted with the management slime (Schleim) at his company that he began a quite different career.
I got as far as the phone interview with Empire. They were looking at putting me in a captain position in an ATR. However, the wages were low enough and the area expensive enough that the pilots there sleep in their cars or the hangar, and use thier outstation to get a decent nights rest...and live at the outstation...in the hotel. The wages were too low to live in the area. Company sounded okay, but wanted a long contract for training, and there are some nightmare stories out there.
They were quite coordial to me, fiendly on the phone, upbeat, positive. I'm sure the job would have been fine, just very underpaid with very little flying...half hour each way each day. If I were single and younger, maybe.
Corporate is based in Billings, an is hiring regularly. Both Empire and Corporate are looking for pilots, presently.