You guys can build the benches however you want. If someone picked a catcher early (Joe Mauer for example) they can use their next pick on Brian Mccann if they wanted. I wouldn't say that it'd be wise, but you can do it. You guys can build a bench however you want (the positions don't have to be a certain way) and you can put players in whatever positions you want.
For example, I can use my bench to use a backup catcher, and 3 OF's, while you might use a backup catcher, an infielder, outfielder, and the best player available. The only thing I would like to see is everyone have the same amount of pitchers and hitters (ie 12 pitchers and 13 batters)
From there, you'd probably have 5 starters and 7 spots in the bullpen, but you can build the bullpen anyway you want, lefties, closers, set up men, whatever. Just keep in mind that you'll end up picking a closer out of the group.
The DH's don't have to be DH in real life. Going back to the first example, you can have Mauer and Mccann on your team, and you can use either as your starting DH or your backup catcher.
In fact, I ENCOURAGE you to build the best team you can. You can have all starters on your team and use them in bench roles.