I'm currently in a game as Brazil in about 1850 and immigration is no problem at all. Plurality is about 70% thanks to the liberal revolution (which gives you +10% overall), having a liberal party in power (and therefore the +25% from events), and putting a socialist party in power before they got voted out in the next election (another +25%).
Also have all reforms but the work hour one (which reduces efficiency).
Also have all reforms but the work hour one (which reduces efficiency).