Yup, you could put in an atlas for the starter mech, but would you really want to? In the early game, you're going to be missing a lot of shots since the pilots aren't going to be very accurate. Then repairs are going to cost you more than you're going to make on any of the early missions. You're going to go broke fast that way. I'd be tempted to put in a T-bolt as the starter mech, but that would make it all too easy in the early game, as I'd probably take a lot less internal damage, and be able to more quickly finish off the opfor. But again that feels like cheating (which it basically is), and I'd rather be forced to learn the hard way to better my tactical game so later on I'm a tactical genius on the level of Mitch. ....no wait, I'd rather actually win some missions now and again. A tactical genius on the level of Eck.
So long as you don't get AJs luck.