Fire prime channelers will have only three of the remaining spheres to choose from as their secondary sphere. The Fire/Spirit combination is disallowed, as the spheres are considered contrary in nature. The choice of secondary sphere strength, along with primary sphere strength, will determine the strength of weaves and stamina needed to cast.
For example, it is likely that a channeler choosing Fire/Earth will be able to cast a longer lasting Haste weave for less stamina than a channeler choosing Air/Water, though not as long as one choosing Spirit/Water. Sphere strength and its effect on weave strength, duration, and stamina is complicated, and still being researched by the most experienced players.
Please check the weaves page for the sphere strength of each weave.