Higher numbers mean higher priority.

Yes,set the number higher.As an example, if you name your castes including the priority no, you can see directly how the order is, for the sorting and maybe also add a M for manual, and I for inherit, all others are automatic, which will be the most I guess like
001p_name_M
015p_name
030p_
045p_
095p_name_I
140p_name_M
3 digits until 999 should be high enough.

when you change a priority, remember to change the name too.

This is my way, and it was helping me a lot to understand the priority system.
Otherwise you always have to go into each caste setting to see what number it has.
Leave some numbers free in-between, so you can add easily new ones in-between.

Thank you so much! Just one last thing - if I set the castes up and manage to ban opposite-gender romances for teenagers, will it just prevent new opposite-gender romances or will it proactively break up any teenagers who are currently in an opposite-gender romance?
Also is there a way to just make everyone actively bi? because i've noticed in the past that storyprogression has a strong bias towards het romances
Also, I know this is a long shot, but can I make it so that storyprogression pushes poly romances as well?

When you use SP to ban a certain type of "Romance" for a certain sim or set of sims, what you're really doing is banning romantic progression, in either direction. So no, SP won't break up the existing couples; they'll stay couples at their current state of attachment as long as that SP setting applies. The game itself can break up couples, but that's somewhat rare unless one of the sims has a history of cheating, or "cheating" by the game's reactionary standards.

To make all existing sims, teen and older, bisexual, you'll need MasterController and its Cheats add-on. Click on City Hall and select:

NRaas > MC > Intermediate > Gender Preference > Age > [all ages Teen and up] > [All]

Set the preference for Males to a very large number, and the preference for Females to an equally large number; you'll probably want something in the thousands. The idea here is that even if a given sim gets very invested in one relationship, their interactions with their partner won't be enough to erase the head start they have in liking each gender. By selecting all sims Teen and up, you're applying the setting once to every sim in the world.

In case you don't know, each successful romantic interaction tilts a sim's gender preference +4 towards the gender of their partner and -2 away from the other gender for the one initiating, or +2/-1 for the recipient. An unsuccessful interaction has the opposite effect. So a small preference either way can easily be overcome with a few interactions, hence the need for very large starting numbers. A large number doesn't make sims more attracted to that gender; it just makes it more difficult to erase that attraction.

You can also tell SP to make all newly-aged-up sims bi as well:

SP > General Options > Options: Flirt > Chance of Gay Sim > 0
SP > General Options > Options: Flirt > Chance of Bisexual Sim > 100

The chance of a gay sim is rolled first, hence the need to set it to 0; only sims that "fail" this roll can be bisexual, and the ones that fail that second roll are straight.

For the poly question, the game itself only allows a single official relationship (per world), and NRaas mods don't unlock this limitation. So even if you had a mod from another source that allowed multiple significant others, I don't know that SP could help push the relationships because it wouldn't know anything about the other mod. If you just want multiple Romantic Interests, that's easier—SP won't push it per se, but you can encourage multiple flirts in quick succession and tone down jealousy to your liking.

you also need to change with MC the long-term relationship by category, filter all teens again and change to best friends or something similar.
Maybe you need to do it for each, because you have to choose the recipient too, A and B.