i came across a case where someone called a position first but someone said that since he was first in order he got to pick what he wished. I've only played 5's with a full group so i never had this problem. what is the generally accepted rule for this since it was i am assuming a full group of randoms?

That is a great question, lucky I have no come across a case like this out of the 72 I've done.

Common sense and common courtesy.

Neither is a reportable offence. However, sometimes a pick by either of those is a clear indication of harassment or trolling - for example, someone calls mid first, but a higher pick takes it. The first caller picks a mid also.

Usually that kind of behavior will also continue in the chat, and that's what will determine your judgment. Champion select is not something you have much information about in a Tribunal case, so it's not a solid reason by itself to Punish. However, combined with other information, it can complete a picture of a player as toxic.

If you are doing draft mode, pick order has priority simply because if you choose something not needed as a later pick you are hurting your team. It causes arguments and generally leads to bad games and reports.

In blind pick it is call order but this can be ninja'd by instalocking jerks.

At least that's how I've always seen it.

That being said since I can't see what happens in champ select it holds little bearing while judging in the tribunal.

Working with your teammates to create a good composition has precedence.

I'm serious, Riot is quite specifically avoiding making a rule here, because they want people to communicate, negotiate, reason, and work it out amongst themselves.

If you have a Tribunal case that stems from this problem, punish if the person seems like they weren't trying to support their team, and pardon if it seems like they were.