If you get a tiel and the keet make sure you DO NOT cage them together. Quarantine also.
Keets are known for amputating toes, or so I've heard, from larger birds. Also known for bullying larger birds. Tiels ca also easily kill a keet.
As far as personalities go, I have two keets. One normal cobalt and a spangle cobalt. My female (tapa) is VERY bossy! If she doesnt want Tio to perch, he doesnt get to have a perch lol. Also if I have her out of the cage she is usually the last one to get back into the cage. My Spangle (Tio) is a very sweet little guy. He is the first one at the cage door and is always willing to eat some millet from my fingers. He is more tame and LOVES playing with his Ferris wheel toy. Mt keets have as much personality as my conure does lol.
So if personality is an issue for you, dont worry all birds have personality once they've become comfy with their surroundings.
As far as letting keets interact with a bigger bird I would just watch VERY closely. I let mine around Xander sometimes but always right there ready to intervene in case something happens. All birds will react differently so just keep an eye out