1. The priests had to look after their own fellowship with God if they were to minister to others -- suggests that they probably did look after their own fellowship with God and consquently did minister to others.
2. The priests would have had to look after their own fellowship with God if they were to minister to others -- describes a more hypothetical situation where, at the time in question, the priests maybe hadn't started their work, or there was some doubt about how it would progress.
3. If I were president, I would give more money to the workers -- means just what it says.
4. If I were president, I would have done more -- doesn't sound quite right to me; the tenses are mixed up. I think it should be If I had been president, I would have done more.
5. If I can have more people in this project, I will be able to finish it on time -- suggests that a request is being made.
6. If I can have more people in this project, I would be able to finish it on time -- sounds slightly odd to me. Better, I think, would be If I had more people in this project, I would be able to finish it on time, which, compared to (5), is more hypothetical, speculative or even pessimistic about the prospect of getting more people.