I understand the metal shafts can be cut to a custom size. Can the plastic shafts be cut to a custom size?
Please review the Q&A Usage Guidelines before posting, specifically point 3, "Quote the applicable rule from the latest version of the manual in your question." Often, you'll find that by quoting the rule, you'll answer your own question.
All part modifications are governed by rule R3, quoted below with a portion bolded for emphasis:
<R13> Parts may NOT be modified.
a. Examples of modifications include, but are not limited to, bending, cutting, sanding, gluing, or melting.
b. Cutting metal VEX IQ or VEX EDR shafts to custom lengths is permitted. This is the only legal exception to this rule.
Therefore, no, cutting plastic shafts is not a legal exception to R13.