Look into the file producer before you pick up the files. It wouldn't help you that much if you like a tist, but it turned out most of their stuff was financial domination. There are a few that tend to have some free works up first that'll let you get a good taste of a person's work first.
In the works of the stuff I've looked into they usually tag what is in a file or tell you in the description. Tends to help people get their bearings and decide if they like a tist when they're getting into the person's work, so it wouldn't do for a file that says it's a relaxation file to surprise a person because it's also supposed to you convince you to turn over your paycheck. So normally they're pretty good about labeling things, so they don't get problems or complaints from a file containing significantly more extreme stuff than advertised.
I've seen files that are supposed to do that around, but I won't speak to their quality as I generally avoid stuff that would require them. Though it should take you more than one listen of a file to get you from say not wanting to be being into sissyfication to full on sissy slut or whatever. Most of the time just don't deal with the file or a trigger for a while and that normally helps get some of the stuff out of your system if you found something starting to get a bit detrimental.
There isn't really a problem with disturbing a trance early. Really I'd suggest starting with something basic and lightly sexual before going into stuff you're concerned about. Never really considered using a spotter when trying to trance before, so don't know on the necessity there. Plus not really seeing it as practical.