I tell you what little I know. My daughter guided on the Middle Fork for 3 seasons and I have run the MF & NF.
Middle Fork: Roadside Class III, fun rapids, borders Glacier National Park.
Even being roadside there are overnight options but also bear issues.
Two separate Class III sections with a big flat in between and killer scenery. Easy shuttle or hitchhiking options. Above that is the Schafer Meadows section (25? miles) that is wilderness Class III / IV but requires a flyin or pack horses and usually done as a two nighter.
North Fork: Float trip on the western border of Glacier. Scenery & Fishing not much for rapids.
Start in the town of Pole Bridge and float to the confluence.
South Fork: Another pack or hike in with a narrow gorge. Have not done it.
Lower Flathead/Buffalo Run: Below Flathead Lake, need tribal permit, easy to get from sporting good stores.
Dam controlled big water Class III. We ran it at 18-20k cfs and it was Grand Canyon size wave trains. Big fun.
Not sure of the overnight restrictions, but there were plenty of potential camping spots.
One caution for Northern Montana boating. The days are incredibly long, we were boating till after 9 pm some days. We did the Albertson Gorge on the Clarks Fork AND the Buffalo run on the Lower Flathead in the same day! Too much fun!!!