Dand,
The Beeman C1 is a Theoben which is Beeman branded. It is a nice light weight carbine that puts out about 850 fpe. It is a solid well made rifle that is quite accurate and powerful. Of all my rifles, it is in my top three and is one that I always grab when heading out on important hunts.
Regarding caliber and hunting; I hunt a lot with airguns and have for many years. I have hunted in South Africa, Australia, Europe, and all over the States for varied quarry, with a variety of air rifles. I have springers, pcps, and CO2 powered airguns in .177, .20, .25, and 9mm calibers.
Here's my opinion for what its worth; most magnum air rifles that generate over 14 fpe and are accurate - very accurate - can be used for hunting most small game. I have hit jackrabbits, prairie dogs, and squirrels out to 50 yards with a .177 head shot that have dropped them dead on the spot. It is better to have a .177 that you can fire tight groups with than a .25 you can't hit consistently with.
You need to run different pellets through your gun to see what it digests best. I like heavy round nose pellets better than hollowpoints or wadcutters, they are heavy, hard hitting, penetrate well, yet still expands to open a good sized wound channel.
Now I say this within the context of small game, if you are going after marmots, nutria, or larger animals, a larger caliber is better. Practice with your gun, and know what you and it are capable of – then bring it into the field and hunt it! Good luck and have fun.
