Partition suit:should defendant act to implead parties even if plaintiff won't?

It's the case of plaintiff whereas you are defending your own right as defendant, accordingly the plaintiff may or may not implead third party in the suit is not your call 

A defendant can implead a subsequent purchaser as a party to a suit if the court considers it necessary or proper. This can happen in cases for specific performance or to avoid multiple lawsuits. 

If the subsequent purchasers are not made parties, it would result in further complexity in the matter since a simple partition suit has been unnecessarily complicated by the defendants' actions. These subsequent purchasers are proper parties, though not necessarily indispensable

In a partition suit, a defendant can implead another defendant if the court deems their presence necessary to effectively adjudicate the case, typically using Order 1 Rule 10 of the Civil Procedure Code (CPC) which allows for impleading parties at any stage of the proceedings if their presence is crucial for a complete resolution of the dispute; this means a defendant can bring in another party who has a stake in the property being partitioned.

You must do RIGHT NOW. Not later. 

Ideally, the plaintiff should implead the third party. But, if the plaintiff is not doing it, even a defendant can implead a third party as a defendant, because this is a partition suit. However, you must show valid reasons to the Court for impleading the third party, otherwise your petition to implead the third party may fail. So, keep all records ready, and be clear about the reasons, before filing a petition to implead the third party.