root length before transfering to hydroton

I'm thinking they should have several roots several inches long?

never done directly to hydroton so kinda worried about it staying upright...

I would think at least a couple of inches I'm not 100% though. Good question I can't wait to see an answer.

It really depends on the strain, some will take well to transplanting some wont due to the environmental conditions shifting abruptly. Trial and error is your only way to learn what works and what doesn't work for your specific strain. If you know what genetics you're working with i, or someone else might be able to tell you how resilient the plant is.If the first transplant goes wrong and the plant dies you know that you have a sensitive plant and will need to get a more established root system before transplanting. Evaluate how the plant could have died if it did, preventive mearsures can be taken next time around if you know what to look out for.
I transplant my BlueberryGum in rockwool cubes to soil once they have sprouted lateral roots through the bottom, if it is a single long root protruding ill transplant it once 3" long
I follow this guideline with all my rockwool cloning and have had only 1 plant that is sensitive to transplanting Green Crack, which need a well established root system to survive the swap
Good luck !
-CC

If your doing hydroton in some sort dwc as long as the water is touching or 1 -2 inches away you should be fine just , just make sure the roots are as close to the bottom of the net cups as possible. Prop it up in the hydroyon they may lean a lil but as the stem gets stronger they'll be fine just keep it in the middle. Good Luck.

Happy Growing.