For my setup I use 3 temp probes. 2 of them are only an inch long and the third is 4". First one is installed in my HLT in an SS T off the ball valve. I personally use a pump to recirculate hot water from the lower ball vavle back up to the top of the kettle to create a whirlpool for the HERMS coil. So it reads the exiting temp of the HLT.
The second 1" probe is again installed in an SS T fitting, and it mounted on the top of my mash tun where the recirculated wort re enters the mash tun. I don't really care what the mash temp is at the bottom, just what the temp is returning to the mash from the herms.
The third is installed in my boil kettle where the old analog thermometer was mounted. I only use it when I'm heating up water for mash in, otherwise, it's not used.
This setup works pretty well for me. it allows me to use only 1 PID and temp input to control all aspects of the brew day.
by the way, I switch between temp probes with a quick disconnect connection on each of the probes.
hope that helps give you a few ideas.