Agreed, don't worry about it.
It's a valid concern that some people join games and leave them again if they don't find their starting position interesting.
However, by your chat to discuss borders or other relevant game information, you show intent to participate in the game. (We haven't seen the message you started the chat with, so that *could* have been rude, but the way you phrase your question doesn't indicate that.)
If someone doesn't like the way you talk (or, if you don't like the way they talk, which is just as important!), you may have figured out where you should keep your borders strong.

So no, there's no unwritten rule about communication before the game starts. Now go ahead and feel confident to make decisions based on that chat!
