If you're playing a race with the special power "Diplomat" during the first round, are you allowed to declare peace with a player that has not yet entered the game?
Q: Is a race with the Diplomat power allowed to nominate a player, who has just declined or who has not yet had a turn (and therefore has no active race) as an ally?
A: Yes indeed, the only restriction in the choice of an ally is that you cannot choose a player if you just attacked his active race.
Sure. There is nothing in the rules to suggest that the race you choose needs to have units on the board.