automatic slack adjusters

Don't do anything to any adjuster until you have the experience and skill and the appropriate manufacturers info on them. And has been said, all of the parts of the braking system work as a team. Slacks are only part of it.

 

Of course they can be manually adjusted. I'd love to see how you'd get your brake drums off when you had to replace drums, brakes, or wheel seals otherwise.
And the best tool to get is the actual, purpose-built brake adjustment tool. Before I'd bought one, I'd use my pass-through ratchet with a 7/16" socket.. it worked, but was less than ideal. If you have the Meritor type, I personally think a set of sidecutters is better than a screwdriver for lifting the pawl, but if a screwdriver works better for you, so be it.