Does a monk with a monk’s belt and the Superior Unarmed Strike feat benefit from both, or does just the highest benefit apply?
In this case, the better benefit will prevail. These effects technically do not have a bonus, so they would not use the stacking rules. One effect will end up overshadowing the other.
Through the RC kind of lampshades this when it brings up all alternative power sources obey the general stacking rules. And while Feats are not named explicitly, many Feats provide such power sources or magical effects to begin with making it an area that needs DM adjudication in the more grey areas. This ruling is kind of a kick in the nuts for me but I can see where they are going with it, Unarmed Damage can reach silly amounts and by preventing stacking effects like that you not only nerf a needed area but you also bring back value to actually using the Monk Class to make an unarmed focused build.
Now Fist of the Forest is by RAW not that helpful and it's a very limited if-than-else. At the 1st level you deal 1d8 unless you already deal that then it progresses per Monk which unfortunately means if you already deal
more than that it the entry is silent on the issue. A little patchwork from intent and it simply bumps you up per Monk.
And finally you apply the Monk bonuses then Size bonuses, and this isn't a hard rule the controller of the Effects can stack them in the most favorable order but there really isn't any details on a "Huge Monk" progression and Monk is unique in that it can upgrade to d10s even through normal size progression doesn't.