"When I do reach such a situation, I point it out to them and generally develop ties with them. "
If you say anything along the lines of, "hey we are allies in this game, ally with me here or I will stab you in that other game", then you are metagaming. The diplomacy/press in one game MUST stay in that game only.
"If the two of us join a game together after becoming acquainted, and I choose to seek him out as an ally because I feel it would benefit me in game, would that be Metagaming?"
This comes very close to severe/punishable metagaming. If you two are joining a game with the understanding that you will ally regardless of what your neighbors say to you then yes, it is metagaming and is not allowed.
"How about if I feel more inclined towards allying with a player I've played with as opposed to one I haven't because I have an idea of his playstyle and "stab" probability? "
Technically this is metagaming as well, but is fairly common and hard to avoid outside of anonymous games. As long as you keep an open mind about everyone in the game you are probably alright. Keep in mind though, only allying with people you know can severely hurt your own chances and limits your diplomatic options.
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