Nightmare probably. Since Skeelz don't life gap or deal that much damage usually (Xingshu fixed that, but idk if you have her), people usually pair them with clans that have high life gappers without relying on bonus like Nightmare with Oshitsune and Pan.
Not specifically. To see how well two clans synergise together, its best first to look at their bonuses, and then look at their clan's strengths/weaknesses. You want 2 bonuses that compliment each other, instead of being the exact same. For example, Pussycats and Jungo's bonuses compliment, where as running Sentinel and Sakrohm is a little redundant. It's hard to do this with Skeelz, as their bonus is particularly unique.
Then, you need to look at a clan's strengths. I'm not overly familiar with Skeelz, so I'm not an expert in this respect, but they have some true powerhouses and hard hitters. Their focus when Greem was unbanned was all about creating a life gap, but with the meta shifting and cards like Plunk becoming staple, its hard for me to tell.