- Create a deco spawning POI for just the Forest Biome through biomes.xml, or a regular POI restricted to the Forest and tagged wilderness using the rwgmixer.xml
<prefab_spawn_adjust partial_name="speciallumberjackpoi" biomeTags="forest" bias="20" min_count="#" max_count="#"/>
Maybe I don't fully understand your advice, but a Decoration (which can be built like a POI) wouldn't use rwgmixer.xml. It would appear in biomes.xml where you can control which biome it appears based on where you choose to insert your entry, like this:
<append xpath="/worldgeneration/biomes/biome[@name='snow']/decorations">
<decoration type="prefab" name="zztong_hunters_trap_01" onslopes="true" prob=".001" />
</append>
But, if you were to build something substantial and really wanted a POI, then adjusting rwgmixer.xml to specify a biome would be cool. I would point out that messing with bias and using min/max counts is optional.
A bias of 20 is very heavy. By way of comparison, by default a POI has a bias between 1.0 and 2.0 depending on the tier. (Tier 1 is 1.2, Tier 2 is 1.4, Tier 3 is 1.6, Tier 4 is 1.8, and Tier 5 is 2.0. You see a high bias used for traders. A bias of 20 will see that POI placed as 20, 10, 5, and 2.5 before a Tier 5 will beat your POI in the competition for placement on a specific Tile and Marker. (Bias for a specific POI gets halved after each placement.) You can see why they're making the bias so high for traders since their goal is to place many of them on each map.
Messing with min/max counts is an interesting exercise as you can only specify one value for each and that value will likely be great for one map size and not so great for other map sizes. If you pick a minimum too large then you'll dominate and displace other POIs on small maps, for instance.
For both bias and min/max counts I suggest leaving them off unless there's truly a special or unique situation involved. That way RWG has a free hand to use it as needed.
(I hope this isn't outdated information about bias. I'm running on what I knew for A21.)