I used to work with that kind of stuff a zillion years ago. I'm not (especially 30 years later) an expert in this field, so consult the web or an expert friend for a final opinion. Anyway, here's a hobbyist's opinion: I think you need to know the sum of all the voltage drops in your loop, and have a constant-current power supply that is capable of supplying at least a little more than the total sum of voltages (to guard against fluctuations). Your question was confusing and made me wonder if you were using a "constant current" source for your loop. A 24 VDC constant current source is often the supply that's selected, but it depends on whether or not the sum of voltage drops exceeds that value. The CC source will lower the voltage automatically if there are only a few sensors. A regular supply (one that isn't a constant current source) could fry your sensors of course.
I didn't understand the part about GPIO at all.
As far as cloud computing goes, the particle.io people have a cloud service that has some Raspberry Pi clients.
I am the Umbrella man