Do all hoppers work on all paintball guns?
Most systems will fit adjustable feednecks. The only major size differences are .50 caliber hoppers, and the tippmann cyclone hoppers, which have very large feedholes.
No, some paintball guns share common parts, but they are mostly different. All air/CO2 tanks and hoppers will fit all paintball guns, and regulators and macroline adaptors have standard threads which will fit if you have a regulator port. All barrels and some feednecks have different threading which means they can only go with certain guns. Any attachments with mounts must go with the correct size rail. ASA's have either straight or staggered holes.
No not all paintball markers are electric. For instance the Tippman 98', and 98'custom, one of the most reliable and trustworthy guns on the market, are not electric. However buying an electric marker does grant you some benefits. Serious paintballers use electric markers because they allow they to select and customise a mode of fire that best suits them. Some electric markers also have an "eye" which prevents the gun from firing unless a paintball…