Assuming this game is like 90% of the other games currently running, we will use llamaserver. There is an FAQ
here. Since it is PBEM there is no need for URL or port #s. Ferrosol will most likely tell us when the game is up so we can submit pretenders.
Generally, it is considered poor etiquette to break NAPs or alliances w/out following procedures, but there is nothing that can be done to you about it. Simply know that your reputation may suffer.
Trading is usually allowed, at least so far as gems/items go. Trading provinces, especially VPs, is a very controversial subject, and should probably be avoided simply to prevent the thread becoming a flamefest.
Most people prefer to conduct diplomacy and such using forum PMs, or occasionally the IRC channel, since the in-game message system has numerous faults. Absent any specific statement regarding this area, I assume this is allowed in this game.
As for other things to know, I suppose informing you about some common exploits to avoid is in order. If you charm Bogus and Co. you may not copy their orders to other commanders, though you may use them with Bogus and Co. You may not use the spell Life after Death or the Ankh to create hordes of upkeep free priests and/or mages. You should also not "sickle farm", by having an ally cast weak remote attack spells at a thug of yours holding the Sickle Whose Crop is Pain for loads of D gems. These are the most common ones that occur to me, but others may add their own, and as Ferrosol stated, go ahead and ask him if you have a question.