My main piece of advice would be don't worry about being published - just write a really good book, but also don't be afraid to write a bad book. Give yourself permission to fail, and don't be afraid.
The important thing is for the characters to feel real, and to be given the humanity they are due. That granting of humanity is what separates a full portrait from a stereotype.
There's no such thing as a perfect person, so it makes no sense to write a perfect person. I don't know any author who'd try. And we write characters, not representations of groups.
Readers embrace all kinds of characters as long as they are written with emotional truth.
I can honestly say I've never thought for a second about whether a character reflects poorly on any group. All that matters to me is that the character is true to my belief in who he or she is.