How much meat do I need to serve 26 adults?
Generally speaking, a 1/2 lb per person should be plenty assuming you have some food other than meat they will be eating, and it sounds like you do. I'd recommend buying 15 lbs total and you will definitely have enough.

Since you are going to have multiple meats, that makes it a bit more challenging because you have to guess which is going to be eaten more/less. 

This article from The Kitchn would actually recommend a bit less than 15 lbs for you since the meat is not really a main dish on it's own in any of your dishes.

Go with 15 lbs to be safe, and just think through how much of each meat you will need.