HELP, how to set an apparel's "rarity" spawn?

[Gerrymon]

I made a spacer slave apparel for my mod, Slave Outfit. Put a <li>spacer</li> on <tags> so they can spawn on spacer pawns naturally, but now every starter pawns from crashlanded scenario wear that thing D:

How do I make it so some apparel only have a certain percentage of spawning?