As long as Ariados is in play, each player must pay an additional to retreat his or her Active Pokémon. Gluey Slime can't make a player pay more than an additional to retreat a Pokémon, even if there is more than 1 Ariados in play.
Flip a coin. If heads, this attack does 20 damage plus 10 more damage. If tails, this attack does 20 damage and the Defending Pokémon is now Paralyzed.