Nothing creates credibility more quickly. When I take charge of a small team, the first thing I try to do is get everyone involved in doing some work. I show that I'm willing to roll up my own sleeves and that we are going to work like a real team to get some real results. The impression never fails to stick. Focus on Goals.
Goals are more important than processes and systems for a small team.
Set them clearly and early, and then determine whether you'll need any systems at all in order to achieve them. Buy-in is also vital on a small team, where a lone person who has not committed to the goal can create disproportionately large problems. I make sure that goal-setting on a small team is a thoroughly collaborative process where there is complete ownership among all the team members. A tightly bundled team needs a sense of lightness to lubricate work, especially when pressure is high. Where all three of these exist, you're sure to find a high-functioning small team.