And you're right, the Bolians were never firmly established as one of the founding races but heavily implied.
In fact, there was a short, comedy scene cut from the Star Trek: The Next Generation
episode script, "The Chase"
, in which it was revealed by Mot that Bolarus IX was not
yet a member of the United Federation of Planets. The scene was dropped for time, therefore not canon - and later writers did not press ahead with that factoid. When DS9 featured a Bolian as Commandant of Starfleet Academy ("Paradise Lost") that pretty much confirmed that it was in, and probably had been for quite a while.
"The Genesis Wave" trilogy of hardcovers put Mr Mot's family as members of a colony world, perhaps acknowledging the original intention.
* A Bolian crewman (portrayed by Leonard Jones) was seen in Time's Arrow
and a Bolian bar patron in Birthright Part I
* Lysia Arlin, a Bolian shopkeeper who sells jumja sticks on The Promenade, may find Odo attractive. [Deep Space Nine: Shadowplay
* A Bolian (Jeff Austin) appeared in the Deep Space Nine
episode, The Adversary
* A traditional feature of Bolian cuisine is that much use is made of meat that has been allowed to partially decay. [Deep Space Nine: Crossfire]
* Bolian tonic water is a refreshing beverage. [Deep Space Nine: Paradise Lost]
* The human name "Frederick" translates as a Bolian obscenity. [Voyager: Dreadnought]
* When power to the USS Voyager
's lavatories is compromised, Neelix warns to be wary of the Bolians. Obviously, he knows something about Bolians which we don't.
* A group of Bolians help Jadzia Dax celebrate her forthcoming wedding to Worf at a party in her quarters. Although one brawls with Morn, the Bolian and the Lurien end up as dancing partners. [Deep Space Nine: You Are Cordially Invited.]
* A Bolian is present at Quark's when Captain Sisko is awarded the Christopher Pike Medal of Valor. [Deep Space Nine: Tears of the Prophets]
* Morn deposits the proceeds of the Lissepian Mother's Day Heist in an account at the Bank of Bolarus. [Deep Space Nine: Who Mourns for Morn?]
* One of the guests at Ezri Dax's welcoming party is a Bolian Starfleet officer. [Deep Space Nine: Afterimage.]
* One of the Borg victims' memories experienced by Seven of Nine is a Bolian manicurist. [Voyager: Infinite Regress]
* Reference is made of the late Zim Brott's "wife and co-husband". [Deep Space Nine: Field of Fire].
My old Bolian page has not been updated since Geocities folded, but it has lots of other early Bolian appearances listed: