Three more thoughts.
First - I used one of these Fanny Packs for years to carry my wallet, phone, ... quite handy. There were various separated pockets that could sort the film. In fact I may dig mine out for similar needs.
Second - I'd ignore the nay sayers on your film choices. I'd feel very comfortable shooting a wedding with the number of rolls and types of film you have chosen. Last wedding I shot was done with 9 rolls and two types.
Third - With regard to the "high pressure" you mentioned in the title. Relax, weddings are like track meets, there are a few short sprints that are separated by long stretches of waiting (and schmoozing). If you know what's next and where you need to be it's it's easy, just make sure you have a fresh roll loaded before something important starts.