Humans, sadly, are humans; four hundred years and star travel just mean we'll find somebody new to look down on. And how often did we see Quark being underestimated on DS9 because "everybody knows" what Ferengi are like? For that matter, see Mr. DeSoto's reaction to the first look at Romulans in the TOS episode "Balance of Terror". It took the stern words of the captain for him to hide his instant disdain for Mr. Spock. And then there was the way Data was treated when he took command of the ship whose name escapes me, when blockading the Romulan/Klingon border... Yes, sadly, prejudice is alive and well, even in the early 25th century.
I've always thought Star Trek humanity has itself pretty fooled. They say they've evolved past their flaws. No--they have just pushed their sense of the "other" outward, and are now racist to aliens instead of just being racist towards each other. Too much self-congratulation and too little realization of what they've really done.
Partly luck of the draw, partly Grunt finagling to get Roclak on his crew (they met in the Academy), and partly a tendency on the part of a certain assignment officer to dump the losers on the Ferengi, because nobody's ever going to trust a dangerous assignment to a Ferengi, right? (He'll get his come-uppance; I don't know yet if that will be on-screen, though.)
That really stinks. I hope that officer gets what he/she deserves.
As far as discrimination in ranks and assignments...as a Devidian Alyosha can certainly attest to that. Why do you think it took him almost 35 years to make captain? While I think the required time-in-grade is longer in the 25th century due to the increased lifespan for human and other species, let's just say Alyosha had to fight for every promotion he got. "Yes, I am capable of command, NO, I do not eat my subordinates for lunch, THANK YOU VERY MUCH."
(Thankfully he does end up with three backers that I'm aware of, though: Marcus Kane of Starfleet Intelligence, and later on, Admirals Quinn and ch'Harrell--the latter being his fleet admiral.)