to meph's question about hell-linked civs: i'd say give it a shot! layer link things down there like crazy and test it out. however, the only way i know to avoid things created down there from fighting the demons is to give them the 'at peace with wildlife' tag, and i only know that from narhiril's studying of similar trying to get her own creatures installed there from constantly getting killed before anyone got to see them. i swear, lfr manages to be an amazing go-to for creative raw-juggling
also, on civs given spring and such as their active seasons: only those set to spring won't show in the first year. i constantly get visits from modded in races in the first summer, autumn, and winter...and, they usually manage to bring the goblins and kobolds (and other 'surprise' races) with them, meaning all your fun can potentially start your second month of play, if your race active in the summer has a lust for steel or more expensive items to show off. my last game, i had a goblin siege in the spring of year two! talk about a surprise...
if you don't want results along those lines, i believe you can force those civs to show up a bit later by giving them similar restrictions to those goblins and kobolds have for their initial arrivals. also think humans have a restriction, too, as i sometimes don't get them showing up 'til year three or, occasionally, four