The only problem with submitting to agents and presses at the same time is that if you score the agent, you've decreased their potential market by already putting the manuscript in front of the editor. Now, if this small press is so small that they don't pay an advance, the agent wouldn't be likely to approach them anyway.
However, let's say it is a small non-advance paying press, and they want your book. You jump at it (woo-hoo! Published!). You just screwed yourself out of a potentially better deal through an agent with a bigger press. You could always send it to the press (nobody says you HAVE to say yes to their offer), and if they want to take on your project, then you can take that offer back to one of the agents in consideration and ask the agent to help you get a better deal.
Now, how long do you expect these partials to be out? Could you wait, sit on the manuscript while they consider it? If I were in your position, I would wait for the agents. I really want to work with an agent, so perhaps I'm biased, and I'd like some sort of advance. But if you'd be happy with the small press, I don't see why you couldn't send the book to them. Nobody's on exclusive at this point, right?