The selection of species isn't random (assuming that you're on the PC). Just click on the big ass picture of the class and it'll switch you...unless the other classes have reached their limit.
Anyways, I really like the game. I've never played the earlier ones, but it's obvious to me that this one has been dumbed down to fit easier on console controllers. Still, it's fine; it's a lotta fun.
I have a few gripes with the game, specifically the way that the Aliens control...the ability to walk on walls and ceilings is one of the cooler things I've experienced in a game so far this year (the other being Braid; I know, I was late to the party)....but it seems a little broken. I understand that walking on the walls is cool, but when I jump, I don't want to always jump on the walls: sometimes I just wanna jump. Probably a simplification for console gamers, but fairly aggravating all the same.
As for the melee system, I feel that it needs a little bit of...I dunno, explaining. It's not terribly deep, being not much more than a big game of rock/paper/scissors. Light Attacks (LA) are quicker and therefore beat out Heavy Attacks (HA) which take forever to use, but break through blocking, while blocking can block LAs.
LAs > HAs > blocking > LAs
I haven't gotten to play much (maybe 2 rounds of Alien and Marine, one of Predator), but something I did just figure out is that the whole 'hit stun' thing, where you've just been hit by an attack, seems to be purely cosmetic...I don't think that it actually stops you from doing anything. The Marine, especially, is strong in this area: middle clicking (or meleeing) after being hit pretty much stuns them and gives you the ability to kill 'em dead.
TL;DR: Fun game overall, making up for the fact that it's a pure port from the console versions. A little polish could be nice, but hey, watcha gonna do? The melee system is simple, but once we all get the hang of it, I feel like it'll be better.