Here's my barely educated take on air rifles:
First, don't fall for the speed advertisements. Those are typically obtained with the lightest pellet weight and won't do you much good in a hunting situation. My particular rifle shoots Crossman Premier domed pellets in 10.9 grain weight best. Read somewhere the RWS 36 will launch them about 875 fps. The two or three crows I have hit at 40+ yards didn't seem to know how fast the pellet was going.
If I had it to do all over again I would probably go with a .20 or .22 caliber rifle. Again, the speed will not be the 1000+ fps, but the pellets are heavier and would be better for hunting.
Shooting squirrels, you can use a .177, .20, or .22 caliber of any good brand rifle and probably do good. If you are serious about killing a hog with an air rifle you better be serious about how much money you are willing to spend. You could get yourself a 9mm pre-charged pneumatic rifle that would certainly kill a hog. However, you better have a pocket full of money when you go shopping. A gun like that ready to hunt will be $1000, give or take.
Shooting air rifles takes practice, especially the springer rifles. They have a two stage recoil that kills scopes, so shop for airgun rated scopes. Springer rifles don't like to be held tightly, you need to let them do their thing and walk around a bit when you pull the trigger. Also, the barrel time on an air rifle is longer than a regular rifle, so hold your sight picture as long as possible.
Just some ramblings on the things I have learned through the years.