6-link tectonic slam but not on body armour ?

is it possible on helm or gloves to do
4 link: Tectonic Slam, Phys, Melee support, Elemental
+ 2 coming from shaper&elder item ?

Theoretically, yes. Shaper and Elder mods are restricted to certain skills on certain bases and can't roll any arbitrary combination, so you should check if what you want is available first.

Here are the lists for armour based gloves and armour based helmets.

Some may be grouped together, so you should click on the mods to see all available options.