Based on the above bit of Lucilius, I find myself rather doubting the dictionary definition of puellus. Perhaps it derives from a diminution of puer, but I think at very least its connotations are of a boy used as girl, i.e. for sexual purposes.
Cf. this rare deponent verb. All the rest of the words beginning puell- have strong feminine meaning as part of their denotations.
A quick word search shows just how rare this vocabulary is and its sexual overtones.
and the more extensive discussion by Chahoud.