The advice given can be applied to any game, assuming you trust you GM to be able to follow them.
You might be wondering about the difficulty of the encounters listed throughout this book. Are they intended for characters of Novice Rank? Seasoned?
The answer is none of the above. The encounters are created to reflect the natural (or in some cases unnatural) organization of the characters or creatures listed. That means your group had best be warned that this isn’t like certain other games that automatically set the challenge level to something they can deal with. Sometimes a horrific abomination simply needs to be avoided, or clever tactics or hired guns employed to defeat it.
We do this both because it’s more natural and it’s more of a challenge. Besides, any system we create would have a difficult time fitting the nature of your posse. If a big game hunter hires on a squad of expert riflemen to cover his back, it doesn’t make sense hat suddenly every rattler has four buddies as well. That means that just as with a larger party is safer (though it might not fit your posse’s plans to keep a low profile). Of course, it also means the heroes have to feed the Extras and provide food, water, and ammo for them as well. And don’t forget some of these apples are no doubt rotten.
All that said, the Marshal should tweak encounters to fit the nature of his party. You’ll have a good handle on what your party can manage after a few sessions, without the need for some sort of formula.
Almost seems like sandbox style game advice. No charts to balance encounters, no CR ratings. What is a GM to do?
Do what we always did before the charts and CRs were added. Follow you gut and make the game fun.