We are headed to Paris in October, and we plan to do a one-day tour of Mont St. Michel. We would like to take the high-speed train to one of the nearby train stations in order to maximize the time we have to explore. From there, we would like to take a paid tour of the site and possibly any nearby towns before heading back to Paris, if possible. We are looking for suggestions for tour companies that would accommodate that schedule.
You should look at tours from Rennes or Saint-Malo, then. They are respectively 1:25 and 2:15 hours away from Paris by TGV.
If you are on a budget, note that there is a non-high-speed option. SNCF sells a train+bus combo from Paris to Le Mont Saint Michel, leaves at 7:30 AM, returns at 10:00 PM, leaves you with 5 hours (12-5) at Mont Saint Michel. It's a lot of transit vs. time on-site but it exists and is workable.
An alternative would be booking one of the independent tour services which derive from Paris (r/t coach transportation to/from MSM). We took City Wonders' itinerary last month, and were pleased with the convenience and efficiency of everything; and their onboard guide provided a good baseline of history and logistics to maximize our time once at the abbey.
https://citywonders.com/paris-tours/mont-saint-michel-day-trip-from-paris-tour
It's an all-day investment to get to (and back from) Mont Saint-Michel, so if you're thinking of a single day you're likely not going to be able to include other/nearby towns. If you're staying overnight, then the train and independent visit/s would be better.
We took the train from Paris (Montparnasse station) to Rennes. From there we picked up a rental car and drove to Pontorson, spent the night, then visited MSM the next day. It was very easy.
I think if you want to find a guided tour of MSM perhaps you might search to find a tour picking you up out of Rennes. MSM is basically in the middle of nowhere, so I'm not sure touring (guided)nearby very small towns is a thing; Getting to/from Paris and touring MSM would be a full day visit. But you have to consider the timing of the train schedule must coordinate with the tour times.
With a tour directly out of Paris you don't have to worry about finding a tour that works within the time frame of the train schedule.