Can I use Slasher Feat and a weapon with a Whip?

A hit with the weapon Whip is slashing and slows an enemy by 10 feet, and then the Slasher Feat benefit Hamstring upon a successful hit with slashing damage slows the enemy by 10 feet.

Slow. If you hit a creature with a Whip and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.

Hamstring. Once per turn when you hit a creature with an attack that deals Slashing damage, you can reduce the Speed of that creature by 10 feet until the start of your next turn.

Can i stack these together?

Weapons with slow like Whip, Longbow, Club cant be stacked with slow only the first weapon will reduce an enemy speed.

So for classes with Multi Attack, I can choose which one applies the Slasher Feat?

I see nothing preventing the Speed reduction by Slow and Slasher from stacking.

You can choose between the feat or the mastery property which to apply on any attack if you can make many.