It all depends on the Rifle and bipod model.
Yes there are some newer spring air rifles that are capable of being switched to Nitro Piston Gas systems. Check with the manufacture.
Cost prohibitive
No, they must be be recocked after each shot.
No. There are small paintballs mad however for low power spring airsoft guns.
There called C02 air rifles or PCP air rifles
The C02 air rifle that has the longest firing range is the PCP (PerCharged Pneumatic) class of air rifles. These include rifles like the AirForce Condor, Benjamin Marauder and the Benjamin Discovery. But these are expensive air/C02 rifles. They DO NOT use the 12 gram powerletts but require an expensive support system. The next strongest class of air rifles are the Under lever, Break barrel or spring/Nitro air rifles. If you want to understand more about air gun systems then read the link below (It's part of my profile) read the section on "Air Gunning"
Start with the Gamo home page "Gamousa" customer service contact. Or see the link below for shops that repair air rifles.
It a depends on what you mean by "Charged" C02 is the least powerful. Usually less than Spring power. Pump is the next most powerful but is still less than most spring powered rifles. PCP (PreCharged Pneumatic) is the most powerful, it's way beyond spring power. PCP's are usually charged to 3000 PSI and capable to fire a 50 caliber pellet. Gas charged piston rifles are about the same as spring powered rifles
There are several air rifle shops around the country that (TUNE) air rifles. Check the web under" air gun repair" or "air gun tune" and locate a shop near you to have the modification done. Also Pyramyd Air (see the link below) sells a NITRO Piston to replace the spring in most air rifles.
All break barrel spring rifles require reloading after each shot. This is required to to re-cock the spring and reload the next pellet. Almost everyone of them are capable of mounting a scope. Under-lever rifles are designed the same way. The power of the scope is incidental.
Sheridan air rifles who is owned by Crosman Sheridans are 5mm air rifles
Because new model air rifles and guns come out quite often it is impossible to say who is the most powerful. However the top end of rifles are the PCP model rifles. Various makers manufacture the "Precharged Pneumatic" Rifles. These Rifles exceed the C02 and Spring Rifles in power. The Rifle is charged with a Scuba tank. The Calibers can be found in .177, 20, 22, 25, 9mm, 45, and 50 Caliber. Obviously these rifles are more expensive than their counterparts. Several models run into the Thousands of dollars. If you want a powerful spring air rifle look at the "Banjman Super Streak." I am sure that there are others on the market so I would compare the different models for Speed (FPS) Feet per second and pellet size and weight. Several of the new spring rifles can fire a heavy pellet (Lots of knock down power) faster than 1000 FPS.