In my 6 foot tank I have 3 Yellows, a Purple, a Gem, a Kole and a White Tail Bristletooth. All medium size and all of them get along.
Started with the Purple. Added a yellow, the purple picked on it for a bit. Then they were best buds. Then added the White Tail, they totally ignored it. Added the next Yellow and Kole Tang together. The new yellow got and still gets chased a bit but pops right back out of the rocks after the chase to join the pack. The last one added also a yellow was totally ignored by everyone.
The Kole and White tail get along just fine too. , I think the White Tail was annoyed for a while because the Kole followed it around like they were supposed to be a “school” and the White tail was kind of a loner.