You could separate your baby apple snails until they're a little larger.
One assassin is incredibly unlikely to go after an adult apple snail if there are pest snails readily available in the tank. Especially if you only keep one assassin - they're pretty small compared to an apple snail. Mine wiped out my pest snail population in no time on its own, now if I see any pest snails they're never around for long.
Snail eating fish are rarely an ideal solution. Most get very large.
This leaves removing the pest snails by hand as and when you see them. Avoid over feeding the tank and you shouldn't get too many snails anyway.