6 adults traveling, Paris and Brittany, best place to stay?

About towns to stay in Brittany (Bretagne) or Normandy, allowing to easily reach Mont St Michel for a day tour, you have many possibilities. If you consider visiting other places in Brittany rather than in Normandy, you may have a look at Fougères, Dol de Bretagne, Combourg. Or St Malo and Cancale if you wish to stay along the coastline.

I think Airbnb is a good place to search for Brittany. I worry about it as a source in Paris, because they are always trying to shut down unlicensed rentals. You can enter the number of people and number of rooms you need. See if something comes up. I have seen entire homes listed but four rooms narrows it down quite a bit. Bayeux is about an hour and a half drive to MSM. Dinan may be slightly less than that. St. Malo is about an hour. Cancale is about 30 minutes. You would probably find the cheapest lodging in Cancale and could wander from there.

On the south side of Brittany, if you're going there, Vannes is a good base. It's big enough that you should be able to find lodging choices. Auray is very nice also. Much smaller so lodging choices may be more limited.

You will probably end up in a modern resort building and might need 2 apartments, if you use AirBnB.
I'm trying to rent near Saint-Malo and only finding resort apartments that are very small.
In Paris, you could try Citadines or Adagio for legal apart'hotels.
Or, try Accor, Ibis or Mercure chain hotels which are family friendly.
I really doubt that you'll be comfortable sleeping in a Parisien apartment with 6 people. Sleeping arrangments can be very tight and most likely "creative".
If you do find one that fits 6 people, expect it to be expensive and probably in the 7th through the 17th arrondissement.

I think you should look for a hotel in Paris. It will be quite a job to find a four bedroom apartment in Paris. You would be better off trying to find two two-bedroom apartments near each other -- plus that would probably get you two bathrooms. What is the budget for accommodations and when are you traveling? You might want to consider the Hôtel Excelsior Latin on Rue Cujas in the Latin Quarter and the Latin Quarter generally for your Paris stay. If you want to have a kitchenette, take a look at the Citadines properties in Paris as well as Hôtel Résidence Henri IV Paris. I agree with Chexbres about tight spaces for six people in an apartment in Paris. When I used to rent apartments in Paris, I used to look for apartments that said they could house six for my family of four and those were usually the right size for my group and I grew up in a home where we had only one bathroom and one bedroom for me and my two siblings.

On the north coast, St. Malo worked well for us, after having visited Mont Saint-Michel earlier in the day, and then stopping in Cancale for oysters en-route. St-Malo has location both inside the old city walls (intra-murros) or outside, along the coast.

Further west, last year we had a place in Quimper (south of Brest and north of Vannes), a city with an outstanding, historic, indoor market. The annual Cornouaille Festival just ended there yesterday (with lots of rain) - and is well worth fitting into anyone's trip in mid-July.

I can't help you with Paris but we've stayed in a great B&B near Bayeux. There's just the two of us but the Le Ferme du Pressoir has a separate house that would likely have room for all of you, along with the standard B&B room rentals. Go to www.bandbnormandie.com. It's located about 12 miles southeast of Bayeux, just outside Villers-Bocage. We easily took day trips to MSM, about 90 minutes via motorway, Cancale, Avranches, D Day beaches and the American cemetery above Omaha Beach at Colleville-sur-Mer. After MSM and lunch at Cancale, we drifted back along the back roads, stopping in little villages to buy cheeses, wine, Calvados, breads, etc. It was one of our best ever trips to France. We took the train from Paris to Caen and collected our rental car right across the street from the Caen railroad station. We used AutoEurope.