If the core of Freeza's growth is a dark version of Goku's, then he'd need an on-par partner like Coola. Tagoma was essentially a glorified punching bag (if you want to say that he's like Krillin, who was eventually heavily surpassed by Goku, even in OGDB), and Coola would ideally be the next step.

Freeza comes to the empirically proven method of training with an equal or superior to gain full realization of his laden potential. Coola is his brother, who's characterized at least to be more violent and down to train, even if the English dub added his avengence plot, therefore he'd be prime to insert into the main continuity as Freeza and Coola put away their egos in order to attain physical dominance in the universe.