For example, the chicken is recommended at 225º F so if you put the chicken in at the beginning set your temp at 225º F until the chicken is done. When you remove the chicken set the temp to the recommended temp for the brisket or pork for the remainder of the cook cycle.
The other way is to put your least time product in at the end of the cooking cycle. For example, if you had brisket in for say a 14-hour cooking cycle at 190º - 200º F  and you wanted to have your chicken or ribs ready at the same time simply put the chicken or ribs in so that they will be in there for the last part of the cooking cycle. Remember to add about 20 minutes when you open the door. Don't add wood because it's dangerous and you'll still get that smoky flavor we all love using what wood you put in initially.

Always reserve the top shelves for your veggies. We always time their entry into the smoker so they are done at the same time as the meat. For example, if we did our 14-hour brisket and wanted to make some of those fabulous smoked baked potatoes, we'd put them in the smoker on the top shelf 4 to 5 hours before our brisket was done. Same with corn in the husk, smoked tomatoes, squash and almost any other veggie.

 

Summary:
Fish: 

Always smoke alone, no mixing.
Meat:  Pork on top shelves and move product to one side or the other to minimize flavor transfer -- if you want to minimize it -- but we like that pork taste.
Poultry:   Always on lower shelves with mixed product.
Veggies:  Always on upper shelves.