I have found that most insta-buddies are fine as dive buddies, but that I have to talk to them about what I am looking for before the dive. If I expect them to really be available, I use a phrase like "shoulder to shoulder" "within reach" "arms-length" or "a couple of meters." If the dive is tough (deep, cold-water, fast drift, low viz ), I expect the buddy to be within reach. If the dive is easy (follow the DM on a shallow reef dive), I am not as fussy. Sometimes I have to take more initiative to "stick" to them, but no one has refused to do a within-reach approach if I have suggested it. If an insta-buddy did not want to stick close, I would assess whether I was happy to effectively solo on the dive.
I notice that many divers use a "lead and follow" approach, which I don't like: If I am the lead, I have to keep constantly turn completely around to track them; if I am the follower, they are not necessarily aware of where I am. Instead, I prefer "shoulder to shoulder" because I can constantly see them out of the corner of my eye. If I am with a new diver or seriously unskilled diver, I really strongly prefer shoulder to shoulder. With an experienced, skilled buddy, I am more casual about this.
I notice that many divers use a "lead and follow" approach, which I don't like: If I am the lead, I have to keep constantly turn completely around to track them; if I am the follower, they are not necessarily aware of where I am. Instead, I prefer "shoulder to shoulder" because I can constantly see them out of the corner of my eye. If I am with a new diver or seriously unskilled diver, I really strongly prefer shoulder to shoulder. With an experienced, skilled buddy, I am more casual about this.