I mean happiness-wise. Do the two slavery tolerance modifier cancel each other out ?
You can enslave xenos without any problem, at least until ethic diverge, which is basically impossible to prevent with this ethos combo.
Slavery's only benefit in Stellaris is that enslaved pops mine slightly more minerals, and it is also incredibly evil, so I would highly recommend against enslaving your pops no matter what your empire's ethics are. As it stands now, using slavery in Stellaris basically damages your own empire in order to oppress your pops. The fact that so many Stellaris players use slavery anyway... disturbs me.
There's a little more to it than that. Slavery effectively means you can ignore that pops ethics/happiness. If i conquer 5 planets, and cleanse 2 of the bad ones those pops HATE me, and as long as i continue purging, etc. (which i will) if i leave those pops free it's a nightmare. If i cap a planet, and enslave them, set up the tiles, i never have to bother with that planet again (unless the sector ai is acting wonky) No influence lost on intregation campaigns, no down time on mineral/energy production, no additional factions to suppress, so my war machine (which gets bigger, and stronger with each world enslaved) can move right on along to the next empire.
Slavery in stellaris is every bit as much about control as it is mineral and food output (which with slave processing, and stimulant diet is a little more than a "slight" bonus). As a slaver species your own pops will be dedicated toward science, and energy (enslaved pops with power hub 2, are still decent at energy production, not optimal, but good enough, especially with thrifty trait) and you tailor your traits toward that end. You also need less farms to keep them from starving freeing up a tile. Slavery makes decadent 100% free, as your capital building will always produce food/minerals, which is helpful.
Slavery is just a little more hands free i suppose; you're free to do whatever you want as far as policies are concerned.