It depends heavily on difficulty. It feels like you see tons more firebugs and commandos at hard and under while firebugs are mostly nonexistent in HoE. Hard difficulty is still at a point where you can still solo scrakes as a commando or firebug, so they don't really feel the need to change.
In terms of HoE however...
Support and berserker are pretty much interchangeable as the most widely seen perk.
Sharpshooter you pretty much always see at least one of in HoE if you want to win consistently.
You'd see one commando, but not very often. Also, they might change to support in later rounds.
Demos and medics, I feel, are similarly placed here. If there's no sharpshooter, there's most likely a demo with a law.
Then the firebug here. If there is a firebug, they'll only go till wave 5, 6, or 7, and then switch to either commando or more likely support. I think even the people that enjoy firebug understand that although it's pretty amazing at killing trash at earlier waves, the flames might block line of sight from that scrake or fleshpound, which means less opportunity to take care of it before they eat you.