Virtually any bag will fit in the pack. The larger question is, will there be any room for anything else?

Normally, the pack is the last thing you buy; you then get one that holds your gear and has a suspension sufficiently robust to handle the weight of that gear plus the consumables (food, water, and fuel.)

Having said that, which size Aether pack did you get? If it fits you well, that's a good pack (and fit, not brand or model, is all that matters. Don't let anyone tell you differently.) Knowing the size pack will also help us advise you as you go.

As far as sleeping bags, just about any down bag should fit into the pack with plenty of room left over. It should also be able to hold any 20 - 30 degree from a reputable manufacturer (all bets are off if you dip into the bullet-and-bait big box stores, or Walmart.) Kelly, Thermarest, Sierra Designs and others you find at REI or the local shop should also fit.

Finally, in case no one told you, the divider between the sleeping bag compartment and the main pack can be detached at two corners and dropped against the side of the pack. At that point, your whole pack is available to hold it (or, if you bag didn't fill the sleeping bag compartment, it's now open to hold other stuff.) You do not have to divide your load to protect the bag; it will do just fine if other gear stacks directly on it.

Now, about sleeping bags (that might be your next purchase?) There are two types: down and synthetic. Down lasts longer, weighs less, is more compressible and costs significantly more. Synthetic bags are more affordable, weigh more, take up more space, and don't last nearly as long. "Warm when wet" is something you'll often hear inexperienced sales people talk about. It's nonsense. Neither type of bag will keep you warm when it's soaked. The bigger question is how did you let it get that way? It's hard to get a bag wet when it's in a stuff sack inside your pack which has a pack liner, pack cover, or poncho protecting it in the rain. You might get some dampness on the bag inside your tent (assuming it has a decent rain fly), but that shouldn't be a big problem. The way you get it wet is to leave it on a rock while you take a day hike, and the rains come; or you spend considerable time in the water recovering from a dunking, or have to swim a river. Those can be avoided with experience and planning.

Sleeping bags also come in two basic shapes: mummy and rectangular. Mummy typically has a hood; rectangular usually don't. So, for cold temps (say, below 50, based on my own experience) you want that mummy with its hood. The hood adds a lot of warmth (by preventing heat loss through your head), and the mummy shape means you're heating up less dead air space. In warmer temperatures (above 50, for me) a rectangular bag is fine - a quilt is even better. You use a quilt (also hoodless) like the covers on your bed at home: just pull them over you, and let arms and legs stick out as you prefer.

Sleeping bags are only as good as the pad under them - but that's another post; I've probably overloaded you sufficiently for now. smile