If someone with a more powerful mind cast suggestions on your characters, they shouldn't be able to shrug it off. They might be able to resist somehow for a bit but, there is no reason they should be able to ignore it.
Hmm, that's possible. Though I don't know if we've ever used psychic abilities used against Servants before, so I'm not sure what would happen.
I guess I could get a round of opinions on it. What do you guys think would happen?
If we followed Nasuverse logic, Tom's ability would classify as ESP.
Mystic Eyes are a good example of ESP and they do affect Servants, although it would surely be met with a form of resistance according to whom is on the receiving end and what he's actually trying to perform. That could cover a large amount of results but, even if it doesn't incapacitate a Servant, it should at least hinder them in some way. As for if we're going to deduce if Servants are immune to mind interferences, I would say they shouldn't be, regardless of the nature of the power attacking them.
As long as people don't go overboard with their abilities, like "your character is now psychic blank", it should be fine. Not like there's any reason to worry over such matter for now, I doubt any PC's capable of that. (Unless one of them is actually Professor X. in disguise)