Thank you, all, for the valuable advice! I have finally had some time to work on the fusor and here is what I have found:
It is difficult to see in the picture that I posted, but my original turbopump test setup included a right angle valve between the KF-25 cross and the KF-25 tee. I used this valve to isolate the foreline and turbopump from the roughing pump so that I wouldn't suck in air through the oil and potentially foul parts of my system.
As per everyone's suggestions, I re-sealed all the gaskets and connections everywhere in the system and saw notable improvement! At this point, the roughing pump can pull the foreline (now just the cross with no extra KF connections) and turbopump down to 100 microns pretty quickly. There is still a small leak in one of the turbopump connections (I think the 6" CF connection), but it is not large enough to interfere with any sort of operation. I have also definitively found that there are very minor leaks in my barbed hose connections from the roughing pump to the foreline and in the threaded TC to KF connection. Putting some vacuum grease on these points lessened these leaks considerably. As of now, these leaks all together are small enough that they do not interfere with any function. My vacuum system is now a "no worry tool," as Richard put it, for my needs right now. Once I get a capacative manometer gauge and progress deeper into high vacuum territory, I will have a list of things to worry about, but for my demo system right now, this is adequate.
As far as my TC gauge and tube. I managed to nearly zero the gauge, but this was with the turbopump running full speed and given how quickly the needle on the gauge dropped, I do not doubt that this is just the TC inaccuracy some of you pointed out. This turbopump experiment has certainly prompted me to look into investing in higher quality gauges sooner.
I have encountered a problem, though, with how I am connecting the turbopump to my main chamber. Because I was apprehensive about connecting the turbopump directly to the chamber, I bought a length of flexhose like this one: https://www.ldsvacuumshopper.com/2flmeho18frl2.html
(but for much cheaper on eBay) to connect them. Unfortunately, when I connected the turbo to the chamber in this way and pulled a vacuum with the roughing pump, the hose would contract nearly two inches and pull the turbopump closer to the chamber. When I vented the chamber back to atmosphere at the end of the test, the hose would lengthen back to its normal size. This is obviously not permissible. Is my solution, then, to remove the flexhose entirely? I would need to use a straight length of pipe to attach the two, is there a better way that would avoid adding extra connections?
Thank you again,