The character Martia in Star Trek VI is very similar to that of the founders, is there any indication that she and her species (The Chameloids) are related to the founders? Could she have been one of the 100 sent out?
KIRK: How does he manage to change form at will?
CORY: The people of Antos taught him the techniques of cellular metamorphosis to restore the destroyed parts of his body. By himself, he later learned to use the technique to recreate himself into any form he wished. The first time we knew about it was when a guard, seeing what he thought was me in Garth's cell, released him.
TAS "The Survivor"
KIRK: There's something wrong about this room.
(He goes over to a biobed)
KIRK: All right, Winston. You can come out now. I suggest you show yourself, Winston, or whatever you are.
MCCOY: Did you say I'm a man of curious habits? Jim's talking to a table.
SPOCK: I don't think so, Doctor.
KIRK: There used to be only two examining tables in this room. Now there are three.
SPOCK: I just realised that.
KIRK: This is a vial of Orientine acid, Winston. It will burn through almost anything but this crystal. If you've never seen it work, I'll demonstrate on you.
(The table transforms into the alien)
MCCOY: I saw that, but I don't believe it.
SPOCK: A Vendorian, Doctor. Their planet is quarantined, and few people ever do see them. Their ability to rearrange their molecular structure into anything with the same general size and mass and their practise of deceit as a way of life puts them off limits.
With those experiences, the following dialog in Star Trek VI: The Undiscovered Country seems a little odd:
McCOY: Would you mind explaining that little trick you do?
MARTIA: I'm a chameloid.
KIRK: I've heard about you. 'Shapeshifters', I thought you were mythical.
Kirk should have known about other shape shifters already.
So this may add a couple more shapeshifting species before the first chengelings in Deep Space Nine.
Nope. Aside from the fact that they can change form, there's no major connection between the Chameleoid species and the Founders. Notably, the Chameleoids are restricted to maintaining their eyes in order to see the world around them whereas the Founders can evidently use another sense (possibly some form of limited telepathy?) in order to see without needing eyes.
Per this interview with actress Iman from the magazine Cinefantastique:
The only common denominator that we have is the eyes. Everything else I can change about myself but my eyes
A Founder would not have this limitation.