Fairly straightforward because there was minor confusion and I couldn't locate a definitive answer. Does R2D2 have to take C3P0? I think the answer is "no", but any references would be helpful.

Nothing in the rules say Counterparts are mandatory, so they are not.

Additionally, if they were mandatory, then logically they would have no points cost since their points would already be included in the main unit.

If C-3PO was mandatory, it would either have a general rule in the RRG entry or a specific rule on R2-D2's card indicating such. Neither of those things exist, therefore he is not mandatory.

FFG poorly worded the reminder text for Counterpart. I initially thought he had to be included, simple because the text says he "must be added to an R2-D2 unit." I feel something along the lines of "can be added only to an R2-D2 unit" would have cleared up any confusion.