To apply consistent symbology to each of your TIFF files, open the properties of one in your table of contents, go to symbology, apply manual classification, and define the break points for the 6 classes you want to apply to your dataset. ArcMap will allow you to create breaks that fall outside the range of individual TIFFs, so it's not a problem if they have different data ranges.
Once you have your symbology classes defined for the first TIFF, right click on the layer in your TOC and Save as Layer File. Then for each new TIFF, open up the Symbology tab in Properties and click the folder icon at the top right to import symbology (circled in image). Click the folder icon in the window that opens up (circled in image) to browse to the location of the layer file you saved in the previous step.
It will be a little tedious to do this for each of your 143 TIFFs, but it will definitely work.