1 hour layover in Paris (CDG) - Lisbon to Tivat, Montenegro

https://easycdg.com/passenger-information/connecting-flight-connections-paris-cdg-airport/where-to-go/

70 minutes does seem to be the minimum allowable.

If you know which terminal(s) your flights use, you can explore the 'how to' on the above website

Lisbon Airport is prone to delays due to capacity issues. Is there any way you can get a longer connection?
Otherwise if flights are on time, 70 min is plenty.

Unfortunately there’s only one flight out of Paris to Tivat per day it seems. Next one is the next day.

The flight out of Lisbon is at 540am - Friday. So maybe the capacity issues wouldn’t be a hassle that early perhaps?

If it leaves at 5:40 have you considered just flying to Paris the night before?

Ideally we would rather not have to cut into our time in Portugal and then add in costs of travel and hotel arrangement in Paris.

My understanding is that since Portugal and France are both Schengen
countries we don't have to go through security or immigration. And
we'll be able to walk/shuttle straight to our new departing terminal,
where ever it may be in June 2020. Is this correct?

No security, but emigration since Montenegro is not in Schengen.

CDG is a huge airport, but they do have a people mover which whisks you between terminals. Do study the airport layout as soon as you find out what terminals you’ll be flying in and out of. It can be a rather confusing place for first-timers, but do your research to make it easier.
Air France is pretty good about on-time flights in my experience and they generally don’t offer impossible connections.
Your first trip sounds awesome...one end of Europe to the other!

Based on AF website and CDG's website: Air France incoming from Lisbon lands at Terminal 2F and Montenegro Airlines departs from Terminal 2D at CDG.

https://easycdg.com/airport-guide/travel-between-terminals-move-around-paris-cdg-airport/on-foot-terminal-2/

Sounds like we'll be okay even going through passport control.

We booked our flight last night via AF and we chose seats at the front of the cabin just in case we need do a jaunt.

Thank you for the responses - we're excited for our trip!

I think you're right in assuming that a 5.40 am flight runs less risk of being delayed. So... I wish you a good trip then!