do they stack ? because i see the same speed on enemies when i do enough criticals and hamstring lands

I guess not.
I think that Harmstring, Critical Slow, Harmstring Shot, Freezing Strike, Slow, Frost Bolt.


And everything else that give slow don't stack (unless Block Wind Walk and "Normal" Slows)

Slows do not stack. Ofc, excluding the multi effect buffs. For example the vortexes that decrease more stats.

There is a nice clue in l2calc. Go to the slow debuffs, and you'll see most of them are in one column.