Looking for private guide or small tour group

Lin, nobody else has answered so I will.
I don't know where "Hill Country" is, however Switzerland has some of the highest wage levels in the world, and almost certainly higher than Hill Country. Hiring a private Guide will not be cheap.
I would suggest you start by contacting the official tourist sites of the areas you want to visit. They often have guided tours you can sign up for or book as a group.

The Rhine Falls are in Kanton Schaffhausen, their tourist website is: https://schaffhauserland.ch/en/home.html
Half way down on the right you can click on "Guided Tours for Groups".
For example:

Guided Rhine Falls Tour
Price/Group: CHF 195.00
Language: German, French, English, Italian, Spanish
Group size: up to 20 people
Duration 1 hour
Meeting Point Info-Shop Rheinfall, Neuhausen am Rheinfall

That being said, and if you want a guide or tour who would I be to try to change your mind, but for me the Rhinefalls is very much an easy do it yourself activity.

There are trains from Zürich right to the Rhinefalls and then there are boats, and it all pretty easy to do. There are also trains just down (up?) the Rhine to Stein am Rhein which is a nice riverside town. https://wild-about-travel.com/stein-am-rhein-one-of-most-charming-towns-switzerland/

I suspect Hill Country is referring to the area west of Austin and San Antonio Texas.

https://en.wikipedia.org/wiki/Texas_Hill_Country

To the OP, if you do decide to do a tour, try not to book thru Viator which is a 3rd party tour re-seller owned by Trip Advisor. If you do enough research you should be able to figure out the actual tour company and book directly with them. Viator charges either the client or the tour company for their service so either you pay more or the actual company makes less on each person.