I had a lot of thought and deliberation about which races to include in the rulebook as playable characters. I finally settled on an even 8 that I thought would be good for PCs.
They are separated by Citadel Races, and Non-Citadel Races, and there is a reason for this, I will explain.
One of the goals with this project is to immerse players in the Mass Effect universe, as apposed to making a PnP version of the video game. I want to add an effect of realism to it that gives players consequences based on their choices. Race being a big one. The Citadel races are safe choices in the Mass Effect galaxy, they are the standard. They are widely known and 'mostly' trusted. A race welcome in the Citadel is welcomed most anywhere.
Non-Citadel races are another story. These are those who are generally mistrusted by most people, and have a lot of shady past and history. I understand as much as anyone the appeal of being a Krogan, or a tech-savvy Quarian, but there may be consequences to that choice. For example, going into the Citadel may need a special clearance or passport, or cannot go in unless constantly accompanied by C-Sec. They will be a challenging character to play, but their challenge may be what they are. That is one of the ugly things about Mass Effect, it has racism to the extreme, I want players to feel that. Of course on the other end of the spectrum, a Batarian will feel much much more welcome in Omega than a Human or Salarian.
Elcor: Imagine playing a cow with a human brain. ALL the physicality of a cow, including speech, movement, dexterity, muscle range, etc. But the intelligence of a human. Imagine dealing with that as a player or GM.
Hanar: Like the Elcor description, imagine a jellyfish with intelligence.
Volus: There's a reason these guys tend to avoid combat. I'm not one for making all gaming sessions a constant combat grind-fest, but by simply being a Volus, there is a huge liability. By their very description, the pressure suit necessary for survival outside their home planet is essentially fragile. As soon as there is a crack in the suit, their body splits open, and they die. You could have a heavily armored Volus, but as soon as any damage gets past that armor, you die.
Geth: While playing a character like Legion could be awesome, there are many complications of creating a robot character. They cannot be treated or built exactly like an organic character. It breaks from the uniqueness of being a robotic character.
Vorcha: Arguments could be made to making them work, but they just work so much better as a monster race.
Yahg: Again, monster race.
Maybe some of these could still work in a later book (like robot character rules for making a Geth), but in the core basic book project, the stable 8 will do fine.