Greetings, and welcome to CD. You have asked a lot of questions in you post. I'll do my best to answer as many as i'm qualified to answer.
can you use the drive motors for raising and working the arm???
Yes, you can use the motors for anything you wish. My team is using the 4 large motors to run 1 wheel each. That leaves us with the servos to run an arm. I have seen several teams using two large motors to drive with (one on each side) leaving the other two "drive" motors to run anything else they wanted to put on their robot.
which is better/more powerful, the servos or the FTC motors??
it depends on what you what the motors/servos to do. if you are looking for power, or if you need somthing to turn more than 180 degrees, the motors are your choice. Servos only turn ~180 degrees and are far less powerful than the drive motors. Servos do have some advantages over motors. They return their "center" when releases and they can tell where they are in relation to that center.
hi, please explain to me what is the max. the servos can hold?
I believe the servos are rated for 61 oz/in of torque. That means, it can turn with a weight of 61 oz on an arm 1 inch long. As your arm gets longer the weight that you can put on the end of it decreases.
Feel free to ask more questions, but let me point you to the link to the FTC
manual. This can answer any questions you have about, what you can or can't do with the kit in building your robot this year.