"Tummo" means different things depending on tradition, believe it or not. It has multivalent meaning. As a method, it is part of Completion stage in some, but not all, traditions. Even amongst one tradition, for example, Tummo may not be a part of a given completion stage.
In the Sarma, Father Tantra practices are not so concerned with Tummo, actually. It belongs more to Mother Tantra practices. Other "methods" of completion stage are Illusory Body, Clear Light, etc. this is according to the Tantras themselves. In the category of Esoteric Instructions, from the Eight Chariots of Practice, for example, Tummo is by no means common to all Chariots, nor even to all sadhanas within any given chariot.
In the Nyingma, as per another thread, it seems that Tummo, per se, was not really part of any Kama Completion Stage. Amongst the Terma Traditions, of course, it's widespread, but by no means universal. Also, it is relegated to a lesser status, in general, from what I've seen, and the Dzokchen Completion Stage practices of Trekcho and Togal take precedence.
May any merit generated by on-line discussion
Be dedicated to the Ultimate Benefit of All Sentient Beings.