For Cradily's "Lifesplosion" attack, do I need to have the corresponding Stage 1 Pokemon in play in order to put the Stage 2 Pokemon on my bench?
No, the Stage 2 Pokemon card goes directly onto the bench, even though it does not count as a "Basic Pokemon".
Source: Plasma Blast FAQ (2013-08-15), TPCi Rules Team (2013-08-15)

Relevant cards

Note: All applicable cards may not be listed below.
BW10_4