Just buy the tablets all the bladder makers sell to clean with.
You can use a few drops of bleach, peroxide and let them set and the flush well.
People freeze the pms in between uses to prevent bacteria and mold growth.
Bottle and tube brushes are good to use.
I have source and hydra pack bladders now. Source you can open the top and HP you can flip inside out.
I just run straight water in mine or find a supplement with no sugar in it.
I fill with clean water, add a drop or two of dish soap. Shank for 10-20 seconds, then let it sit on the counter for however long it takes me to get back to. Then rinses out a bunch until I know for sure the soap is out. I've never had a problem getting sick or having any weird tastes in my bladders.
I've also thrown them in the freezer as well after they've been cleaned, but not always.
Just rinse, dry, and put them in the freezer. I use the versions with a wide mouth opening (i.e. 32oz Nalgene) and always leave the bag open and detach the hose. Make sure all water is out of the hose as well. Never once had a problem with taste, odors, or anything else.