If they have asked you to another interview, then they did not reject you the first time for any reason that cannot be overcome (Such as perception of incompetence, bad recommendations, failing a drug test or a credit check, having a criminal history, lying). If they felt that the result of the first interview was that your were an unacceptable hire, you would be marked that way in thier system or known to the hiring officials as someone not to consider.
Likely they had mulitple people who could do the job and you were second or third choice or they felt you were a better fit for a different job. It is unlikly that a small company would ask you to interview again if they did not think it was a possibility that they would hire (no guarnatees though, you don't know how good your competetion is).
If they asked you back for a different position, you may be in a good place, often when we see someone who we think would be better for another opening we have coming up, we might interview that person before advertising the position to get it filled right awy.
Go in with confidence and do your best.