Not entirely sure on the TBR7, but for the Hawk it's fairly straight forward. Stock with a 15/50 is 130 links That tends to work ok down to a 17 front and 45 rear. When ran a 43 rear I found a 128 links worked much better. 128 may work even with a 17 and a 40. With a 15 and a 40 though, 126 links may work a little better.
Those are just guesses though. The best thing you could do is order a chain you know is a little too long and then use a chain break to shorten it to what you need.
As far as putting a new chain on, yes it is ideal to slide the rear wheel forward to setup your chain. The stock chains don't have a master link so you will have to either break the chain or remove the rear wheel, and then the swing arm so you can thread it through the chain. Obviously breaking a chain is easier.
A good chain tool kit is a good investment to have if you don't have one. Some of the more affordable ones are made with cheaper materials so if you don't use them intelligently they will tend to break or have other problems. The best thing you can do to break the chains is to grind the riveted ends off of the pins on the outer link before using the press on the break tool. I have a Pit Posse tool that I have done dozens of chains with and had no issues.
https://www.amazon.com/Pit-Posse-Mot.../dp/B004MS9VTW