CO Presidents Club membership gives you access to all NW and DL clubs. It doesn't matter if you fly SkyTeam or any other airline. At some airports you may not be able to access the club because the club and your gate is behind different security checkpoint. For example, if you fly CO from Orlando, you cannot access DL club. For the same reason, if you fly AA, the CO (or DL, NW) membership may be worthless. It is possible to get from AA gates in Frankfurt to DL club in the other terminal without leaving secure area, but it's a really long walk and train ride. You may be better off with Admirals Club card or PriorityPass, which is way more flexible in Europe than any US clubs. RCC club will not get you in any StarAlliance clubs operated by other carriers if you are not flying that particular airline (you must fly LH to get into the LH club with RCC membership card.)

There is free alcohol in all CO and NW clubs (I rarely use DL, so I don't know). In some clubs the alcohol is not served on Sunday mornings.

Snacks are somewhat the same, but I really like yogurts and small cans of juice at NW clubs.

I hope this helps.