You can make up races and an origin planet. That's fine. Right now, the big thing is not so much your race as it is your character. Once you begin roleplaying your character, the culture of his/her race and other tidbit information would naturally come out over time.
If you begin creating more characters of this race or interact with others to the point where your race's homeworld comes into play, then you might want to think about the broader themes of the race and whatnot.
The Atlantean kid in question did not really have a problem with his character, but, rather his 'faction'. If you are not starting a faction on TRF, then you don't have to worry. Typically, if you eventually are ambitious enough to try to start a faction, that's fine. A person doesn't have to start one off right when they come to TRF. Rp your character for a while, get a feel for things.
No one is going to bite your head off.
And don't let the psuedo-angst get to you. It may read like anger but it's more heavy sarcasm, general dickness, poking fun or pushing buttons to see what you're all about.
Typical of any family.