A friend of mine ran #00/3 from his house to his garage, but only wants a 100a service. What can I use to hook up #00 to a 100a main breaker, because I don't think the physical size of the wire will fit under the lugs in the breaker. Also I plan to run a #6 to a ground plate buried 3' outside the garage. Is this suitable for a ground.

They make cable reducers,such as the Burndy #AYPO2/0, shown in the picture.

These need a specific crimping tool that may be rented out by electrcial suppliers, etc.

Or if this is not practical, and if there is enough room, you can cut the #2/0 short in the box, and splice smaller wire on using split bolt or insulated "bugs" to reduce the wire.

A GE 100 amp breaker can handle #1/0 wire, check your breaker to determine the largest wire it can handle. This info is listed on a small label on each breaker, including the proper torque specs. Yes, wire connections need to be torqued to a certain value, and a torque wrench with the proper driver tip is needed.

The ground plate is fine, minimum depth is 30 inches. The plate needs to be no smaller than 2 ft square, at least 1/4" thick for ferrous metal, .06" or 1.5 mm thick for non-ferrous metal.