Is there a way to hide items in tech tree...

Depends what you are trying to do with armor X.  If you just want it floating around in space just remove all tech requirements from it and don't put it on a loot table.  Then give it some nonsensical trait as a prereq so nobody can train it.

If you are trying to make Champion only gear then its a lot easier.  Remove the tech requirement and set it to champion only by adding that flag. Then find a building that you would like to sell that item and have that building unlock at the appropriate tech level, thank Heavenfall for explaining this one to me.

If this doesn't answer it let me know, I have been messing with the techs pretty extensively and can flesh an answer out better with some more details.

Can't think of a way to do this. You can hide units but that's pretty much it.

I think you can do it by having the tech unlock another tech, and have the armor require the other tech. The other tech would be hidden (not in the tree, prereq some nonsensical tech). So basically, Training would unlock armor X tech, and the armor x would naturally require armor x tech.

Hmm not sure that would work as for techs to be recognized in the game they have be in both the tech table and the techtree and once there they are placed into the different areas automatically.  So they would just be a floating tech that would autolink to the previous tech that is unlocking it. However, if you had a tech unlock a trait that unlocked armors it should work in that regard so very similar to what you were going for.

Nah, that's the trick I've been using in my mod for my custom factions (new tech tree) to unlock group techs. It unlocks the old tech that does it even though those techs aren't in the new tree. The tree only determines what you can research, for techs that you unlock it doesn't need to be in the tree.

Interesting, I will have to try this out.