Carbunkle is in most FF's. Shoat and Golem were in FFV and FFVI


Siren from FFVIII was a boss in FFV
The brothers from VIII appeared in FFV as a random encounter and as a boss (When you defeat the random encounter, he says his brother in the tower will avenge him)
The sisters from FFX were a boss in FFIV
Gogo appears as a boss in FFV and as a character in FFVI