Can you solder your tube or get it welded shut? That would be the ideal. You might be able to make a nice clean seal in the tube that hopefully wont hide nasties but if the option is available I'd get it welded/soldered shut if it's stainless.
Other than that I would suspect that it would be fine. There are no high temps to deal with in your fermenter and it shouldn't leach anything. It would just be the sanitary side of things that I'd be more concerned about. I'd aim for the smoothest surface of silicone you can manage.
Just for MHO, I think measuring the temp in the fermenter is a little bit excessive, you can brew good beer measuring from the outside but it's totally up to you.
Good luck, Justin
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