Is it possible to specify different appearances (for example, beard styles) for specific castes/species?
Yes, it is possible, all appearance modifiers are caste-specific. I recently managed to make it work in my mod

Tissue style units are kind of special thing used by civilized creatures only, so you need two things:
1. Select tissue layer group and define [TISSUE_STYLE_UNIT:...] for your creature/caste,
like this:
[SELECT_CASTE:MALE]
[SET_TL_GROUP:BY_CATEGORY:HEAD:HAIR]
[TISSUE_STYLE_UNIT:HAIR_M:STANDARD_HAIR_SHAPINGS] last argument here seems to be irrelevant, overwritten by entity tissue style
[TSU_NOUN:male hair:SINGULAR] TSU_NOUN is not the same as APP_MOD_NOUN, "male hair" is for testing purpose here :)
[SELECT_CASTE:FEMALE]
[SET_TL_GROUP:BY_CATEGORY:HEAD:HAIR]
[TISSUE_STYLE_UNIT:HAIR_F:STANDARD_HAIR_SHAPINGS]
[TSU_NOUN:female hair:SINGULAR]
First argument after [TISSUE_STYLE_UNIT: is an ID used as referrence in entity TISSUE_STYLE tokens
2. Add
TISSUE_STYLE token to the entity and add TS_PREFERRED_SHAPING:X and TS_MAINTAIN_LENGTH below selected tissue style unit.
like this:
[TISSUE_STYLE:HAIR_M]
[TS_MAINTAIN_LENGTH:0:50]
[TS_PREFERRED_SHAPING:CLEAN_SHAVEN]
[TISSUE_STYLE:HAIR_F]
[TS_MAINTAIN_LENGTH:100:NONE]
[TS_PREFERRED_SHAPING:DOUBLE_BRAIDS]
[TS_PREFERRED_SHAPING:NEATLY_COMBED]
[TS_PREFERRED_SHAPING:BRAIDED] Now you have your males go always with clean-shaven hair, and females have either double braids, braids or neatly combed hair!
adding several TS_PREFERRED_SHAPING makes the game randomly choose one of them for each unit
I suppose tissue styles are applicable to tissues other than hair, just follow the above described logic. For example you can make a monster race which double-braid their tentacles around the mouth.