STANDARD_MATERIALS works, but is applying all the materials available to that list, like skin bones and teeth, even though the bee isn't using them. You could instead use [USE_MATERIAL_TEMPLATE:HAIR:HAIR_TEMPLATE] in its place. That will only add the hair material based on the HAIR_TEMPLATE material template, with the local material ID "HAIR" and nothing else. You then need to use [USE_TISSUE_TEMPLATE:HAIR:HAIR_TEMPLATE], because materials do not automatically become tissue layers, the game needs to know that a) you want to add a HAIR tissue layer and b) the hair tissue layer should use the HAIR_TEMPLATE tissue template definition as a default. Then you can select that tissue as is done in the sheep raws and change the name of the tissue to "wool."
Then you need to tell the game what body parts to apply the tissue layer to. For your question, "BODY_HAIR_TISSUE_LAYERS" is referring to a body detail plan in the vanilla b_detail_plan_default.txt. These are used as shortcuts to apply properties to a creature according to the templates defined there. BODY_DETAIL_PLAN:BODY_HAIR_TISSUE_LAYERS:HAIR is applying the HAIR tissue to body parts according to that body detail plan, but you said that you didn't want to add it to every body part, which that BDP does. So, you have a couple options. Firstly, you can copy (and rename the copy!) the BODY_HAIR_TISSUE_LAYERS entry from b_detail_plan_default in the vanilla bodies to a new BDP file in your mod, and remove the lines relating to the body parts you don't want to add hair to. This will mean youre adding a new file to your mod, but that's not a big deal because they're small files. But if you're only going to use it once anyway, you can instead just define them directly in the creature file. To do that, you can add after defining the rest of the tissue layers (or it will end up being placed inside the body, with subsequent layers being added on top of it!) [TISSUE_LAYER:BY_CATEGORY:whatever category you want:HAIR], which will add the tissue to every body part that matches that category string. You can do this multiple times to add it to several different categories.
you are correct that the hair will be very thick, because the way [TL_RELATIVE_THICKNESS:10] works is its adding 10 units of hair on top of the existing tissues. If you look at the EXOSKELETON_TISSUE_LAYERS BDP you'll see the relative thickness of all the other layers on the body template. [...ARG3:6:ARG2:1:ARG1:2] on the body, specifically. The ARG strings refer to [BODY_DETAIL_PLAN:EXOSKELETON_TISSUE_LAYERS:CHITIN:FAT:MUSCLE] in the creature raws (defined under the vanilla normal-sized bumblebee, which your creature copied as a base template) CHITIN would be ARG1, FAT ARG2 and MUSCLE ARG3. adding the relative thicknesses of those three together, you'd get 9. The relative thickness of the hair youre applying on top of that is 10, which means the majority of the volume of the body part would be hair, followed by muscle, chitin, then fat as the thinnest layer. That could be fine for you, if you want your bees to be extremely fuzzy little guys, or you could reduce that down to 1 to give them an average hair coating over the top of thicker chitin, or any value you think suites them.
Tissue layer thickness isnt defined in [SET_TL_GROUP:BY_CATEGORY:ALL:HAIR] because at that point its already been defined, and youre moving on to describe individual qualities of the in-game description of the tissue, not the actual properties of the tissue layer, which are already defined prior.
You can indeed add pollen to their legs. You will have to go through the same steps;
1)Add a pollen material (I think youll have to create a material template file and define one, im pretty sure no such material exists in vanilla) template. Add this to your creature. This will make it "giant bumblebee pollen" on butchering/shearing/milking returns. Unless you add the line [PREFIX:NONE] to the material after selecting it in the creature file;
[USE_MATERIAL_TEMPLATE:POLLEN:POLLEN_TEMPLATE] - POLLEN is the "local creature ID" your creature material will use, POLLEN_TEMPLATE is template the creature material will copy properties from.
[SELECT_MATERIAL:POLLEN] - POLLEN being used here is the above "local creature ID"
[STATE_NAME:ALL_SOLID:pollen]
[STATE_ADJ:ALL_SOLID:pollen]
[PREFIX:NONE]
That will spit out a generic "pollen" material, which will still be associated with the giant bumblebee since it came from it, but wont show that in its nomenclature.
2) Create a pollen tissue layer. Same steps as before, there shouldnt be any such pre-existing tissue template so you'll have to create one in a new tissue template file. In the template, [TISSUE_MATERIAL:LOCAL_CREATURE_MAT:x] is where the game knows what material from your creature to use. its calling for a local creature mat, so the ID its looking for is POLLEN, but the template doesnt know that until the game initializes and looks at the creature file and asks "Which material is POLLEN?."
3) Then in the creature file you add [USE_TISSUE_TEMPLATE:POLLEN:POLLEN_TEMPLATE] - This time youre adding POLLEN as a local tissue layer ID, based off the POLLEN_TEMPLATE tissue template you just defined. The game knows, from the tissue template, what properties the tissue should have and the material its made of.
4) Add the POLLEN tissue layer to a body part; use [TISSUE_LAYER:BY_CATEGORY:whatever category you want:POLLEN] - This time the POLLEN being referenced is the local tissue layer ID. The game now knows to add a layer of that tissue on top of whatever prior tissue layers make up that body part, or if this is the first tissue added to it, it now makes up 100% of that part and subsequent layers will be applied on top of it.
5) Add body tissue descriptors. Since the material and tissue properties and locations are defined, you can now give that pollen a color description like
[SET_TL_GROUP:BY_CATEGORY:ALL:POLLEN] - is searching all body parts of that category for a POLLEN tissue
[TL_COLOR_MODIFIER:YELLOW:1:ORANGE:1]
[TLCM_NOUN:pollen:SINGULAR]
and the game will choose one of those colors to add to the creature description. "Its pollen is yellow" will be printed in the creature's description in the health tab. You can then make it shearable the same way wool is;
[TISSUE_LAYER_APPEARANCE_MODIFIER:LENGTH:0:0:0:0:0:0:0]
[APP_MOD_NOUN:POLLEN:SINGULAR]
[APP_MOD_RATE:1:DAILY:0:300:0:0:NO_END]
[APP_MOD_DESC_RANGE:10:50:100:150:200:300]
[SHEARABLE_TISSUE_LAYER:LENGTH:300]
This should now spit out a "pollen" glob on shearing alongside the "giant bumblebee wool" yarn.