I had heard a statement and i was wondering if it was true we have to use only the motors that came in the kit of parts and no other ones can be used? or is that statement false and any kind of motors can be used for anything in the robot?

You can only use the motors supplied in the kit of parts. The manual specifically tells us which additional motors we can use. Please refer to the manuals The Robot section.

You may only use motors of the type that came in the kit. There are motors that can be purchased in addition to these and those are listed in the rules. There is some difficulty with teams who are trying to use the old Fisher Price motors and the van door motors from past years. The old FP motor has the part number 9003 while the new motors are stamped 9012. Additionally there is no van door motor in this year’s KOP.

<R27> Motors, pumps, and, Robot Controllers from previous robots can not be used in addition to those provided in the 2007 Kit Of Parts. They may be used as direct one-to-one SPARE PARTS for those provided if the provided part fails or is damaged. They can only be used if they are identical to the part being replaced. Note that the Fisher-Price motor found in the 2007 Kit Of Parts (Part number 00968-9012) is different from the Fisher-Price motors used in most previous FIRST competitions. Only the Fisher-Price 00968-9012 motor may be
used as a SPARE PART for the Fisher-Price motors provided in the 2007 Kit Of Parts.

What makes this hard is that some teams where provided with the 9003 FP motor in their KOP. I believe those motors(if they exist on your robot) need to be changed before inspection. There is another thread on this subject.