I also just picked up some johnson's baby shampoo. Seems like it would be gentle enough to not damage my hair and it's probably as safe as a shampoo as you can get . Will report back if there are any residue issues..
Not sure what a safe shampoo is and I wouldn't worry about damaging your hair... generally most shampoos are designed not to damage hair... But just wait and see how your hair feels after a few washes. You'll know if it's getting washed out or not. Neutrogena's "residue removing shampoo" left enough in that when I squeezed them dry I could hear there was goo still in there. But most aren't so obvious, I've used Head and Shoulders on and off in the past to help with if I get dandruff but after a few washes have to swap back. It becomes noticeable in the tips of my dreadlocks where left over shampoo will build up and stuff will stick to it like dust / lint / towel threads. Pretty easy to see.
T-Gel seems to be leaving some residue in my dreads. Not much, but it's sticky and bleh. Going to try diluting it a ton.
I use dr bronners but do remember it's super concentrated and tea tree can allow dead skin cell to show up as well just like peroxide. I alternate between the tea tree and peppermint. My hair does dry fast due to I live in a desert so that also helps. I also wash every 2-3 days.
I always feel like dreads are ones of those things we (myself included) tend to over complicate. A little shampoo residue isn't the end of the world. Over the years, I've used every "bad" thing in my hair (yucky wax, normal shampoo, hairspray, mass amounts of bleach, accidental fish guts) and you can work with/around/fix most things.
I dilute mine quite a bit. I pour a glob into an empty bottle, add water and shake. I find this also helps with lather as T-gel can be a little odd that way.