You can probably do a small turkey. Never tried though. Also, looks like you can put a turkey breast on it.
No I have done many beercan chicken never a issue that I'm aware of
Its not a square base. It is built level and even. Mine sat nice and sturdy inside an aluminum pan to catch all the juices in which I used to base the chicken a couple of times. The beer can sits in the center of roaster perfectly. The beer evaporates inside of the chicken thus giving it flavor and moisture. It comes out super juicy and flavorful. There are plenty of simple recipes and tips online.