If I have a monster, with a class level, that I am making into an npc/monster for the pcs to fight, do I have to go with the standard stat builds?(15,14,13,12,10,8) or can I create my own using the 15 point buy for stats. The race is a pc playable race. Any feedback would be greatly appreciated.
You don't HAVE to do anything. As the DM it's your job to create a challenging fight, so if doing so means ignoring the guidelines they've given, you're free to do so.
If you'd like to use the guidelines because you don't feel that any deviation from them would help the game, then they generally work fairly well. However, a 15-point buy is roughly equivalent, and shouldn't create any issue for you.
So it's really up to you, but a 15-point buy is fine.