Shulker's generally can't pilot mobs when they have the highest priority in the mob stack. To my knowledge there are exceptions. It might work if you put the smallest size of slime as a parallel passenger on the primary slime. Don't quite remember how priority is chosen for parallel passengers. 3 mobs is starting to get on the laggy side though.




Could be interesting enough to just have the shulker not care its stuck in a slime, then murder it to get out when it detects you because the slime is large enough that the shulker bullets hit it. Slime can't move because its to heavy. Once it splits into the smaller slime, it'll be a normal shulker.