Galaxy S5 has a 4-core 2,5 GHz processor, i7 is powered by a 4-core 3,4GHz processor, more recent. So a single phone isn't more powerful as the I7. OP was mentioning 7 S5, combined with a good usage of the processors, it may be more powerful. But it's certainly not cheaper in energy.
You can mine with any phone that can be programmed to mine, but it's often not worth the energy spent.