It's the wee hours of the morning and although I had this thought much earlier in the day, it didn't really take fruition until the early a.m. So, a quick and dirty version, and then I'll plop links for further information.
Evolving to + Monsters: A level monsters are the only ones that evolve and when you finally get a pair, sacrifice them 1 to 1. If you toss more monsters into the mix, your second A level with go the way of the DoDo and I'll be forced to hold a funeral for the loss.
Leveling Monster Skills:1 to 1 sacrifice, like in evolution, the monsters don't have to be identical, but the skill must be the same. Also, know Skill Level Up is not guaranteed, sometimes you will just get the Power Up message. Say a few curse words and move on, happens to us all.
Changing Monster Skill: You can change a monster's skill or give a monster a skill by sacrificing a spirit. In order to do this, you'll need to find a Sleep Spirit, Lower Attack Spirit, etc. to name a few.
Mutations: M is for Mutations and consensus is just as mixed when it comes to when/how it works. It appears, from what I've read, to be somewhat random. People have repeated the process with monsters B and below, but 1 to 1 sacrifices (just as you would do with A ranked monsters) of varied monsters don't always yield the same result or a result at all.
For more Information on the above topics and other Dark Summoner Sites, check links below:
Phone App Chat Programs:
Palringo has a Dark Summoner Group Chat for those who may have the app installed on their phones.
Happy Gaming Folks!