I have a tomato plant in a container that is lagging behind others. I also have some in Hydroponic setup. At what point can I not transfer this container plant to hydroponic setup?
I do not know what type of Hydroponic growing system that you have, so it is difficult to answer.
If the plant roots fit into your system, you can likely transplant with not issue.
If you all ready have a poorly growing plant, you may want to leave in the container and not introduce to other plants.