You can communicate by whatever means you wish.
You can make whatever deals you wish.
The only constraint is that you are not supposed to exchange information that your current game state does not permit.
For example, you should not say 'the zulu are to my south', or 'the zulu are at x,y'
if either:
- your tribes have not met in the game
- you do not have the tech research capability to exchange maps or knowledge
Probably, to be very strict, you should not even mention that you have met the zulu:
as this provides
- a clue to where the zulu are, for someone who has met you
- a clue to where you are, for someone who has met the zulu
With huge maps, this knowledge is probably more of a big deal than it would be on tiny.
In my other games, we tend not to be strict, and cheerfully call out publically when we meet someone. This at least is public, so no-one profits in private from it.
But also, these games also tend to have unwritten rules,
like we never used to attack each other at all,
or at least until all the ai were gone,
or if the game was always war.
However, part of the charm of Upsilon for me is that I am playing with new people,
so there are no unwritten understandings from past experience.
So, for example, I made some general comments about my two tribes geography,
but these were general enough observations to not be directly useful to anyone.
In the early games I played, we made no contact or plans with other leaders, until our tribes had made contact in game. I would probably stick to that for anything concrete.
(many hours later sorry) 
