No, you cannot go into decline immediately, because selecting a new race includes a "first turn conquest". First turn conquests does not allow for the option of declining. This can be found implicitly in the rules.
Edit: With the Stout power, you could of course decline during the turn in which you selected the race.
Thanks for the clarification, this is the way we played since we couldn't find more clear rules quickly, and I still won so no harm, no foul ;-)