I don't think Trans Fix will harm anything. It's just a bandaid that kicks the issue down the road. Sooner or later it's going to need fixed and possibly cost more to fix since the needed repair has been delayed.
I will add a quart of ATF and some trans fix?
If you don't have the money to fix it now, Trans Fix will keep you on the road and give you time to save up the money needed to repair or replace the transmission. I have the money but the Ranger is really a back up vehicle due to the frame being corroded to the point of not on the top of my priority list, but if I can get it back to just local running around that would be a gain. See picture of frame on passenger side where top strut mount. This is why not pressed.