You have to post all the datasheets or vendor links for everything except the arduino. That means all the hardware you want to control. We need to have a schematic (a photo of a hand drawn schematic will suffice) , the datasheets and vendor links and the criteria for operation of the device, including tolerences. We need to know exactly what you are planning to use for a power source
and that needs to be shown in the schematic. A thermister is a very poor choice for a temp sensor if that is what you were planning to use it for. I don't know what a TEC is so you'll have to explain where the temperature comes into the picture.
I don't see what temperature has to do with the laser. It is either on or off , right ? Are you talking about controlling the intensity (brightness) by controlling the current of the laser diode ? For what purpose ?