Start with the English, so you get a Brig rather than a Sloop.
Spanish cities will not spawn Ships of the Line.
What I like to do is 'join' one country, and just beat the hell out of the other three, until you get your highest rank, and then move on. In my game right now, I'm privateering for the Spanish, because that allows me to get at Baron Raymondo faster. But, now that I've got my Ship of the Line and I'm a Marquis, I'm going to become the terror of the Spainsh Main!
Oh, and don't be afraid to attack the occasional Treasure Fleet or Military Payroll from the country you're fighting for, they really won't care too much, so long as you don't make a habit of sinking every ship in sight.