While I certainly admire creative ideas such as your poor yahg, I'm going to err on the side of canon caution and say no, for the reasons Tick mentioned. It would also be really hard to take a yahg anywhere since nobody really knows what the hell they are for the most part, and I can't imagine the salarians allowing one to escape. The Shadow Broker was a very special case, and he was surprised and angry Liara knew what the hell he was. If you wouldn't mind, stick with one of the regular pre-ME1 races. 

It's not to say we couldn't eventually work something involving STG insanity and maybe even an Axis cameo down the road, but it's rather tricky to justify more than that. I appreciate you checking first! I don't like turning down people's ideas when they're pretty reasonable, but given our timeline and how we're trying to stay fairly true to canon, I don't think that one would work in this game. Sorry!