a jardini, a fish known for not allowing tankmates and a killer, placed in the same tank as a rbp big or small will equall almost certain death.
theres no way to spread the aggresion of the jar with only one target fish(the rbp). a small rbp will almost certainly be killed after it has chewed the jars fins to crap, as juvi rbp's eat fins for nutrition in the wild and captivity.
a large rbp and a large jar will just battle royaal till something is dead. yeah i know rbp's are skittish but theres nothing more dangerous than a rbp who is scared, they just bite as a defense.