You'll need an adaptor to go from soda stream to the standard DIN477 / W21.8-14 / BS341 #8 thread you'll find on most industry regulators. Personally I'd rather go to 'proper' cylinders and just screw that regulator direct to it, but the difficulty in obtaining cylinders might put you off plus the size of them depending on what you've bodged it into.
An adaptor like ...
https://www.ebay.co.uk/itm/Soda-Str...ome-Brew-Beer-Keg-Tank-Regulator/312684905209
Will allow you to use that regulator with a soda stream bottle. You might also find you don't pay much more for a little regulator designed to work with a soda stream bottle (they are very common for aquarium usage) which would give you a 'rough' read out of the gas pressure left and probably contain a needle valve for rough adjustment. You'd still need secondary regulators though if you want individual controls.
On the topic of individual control and that particular regulator you've what looks like an unregulated direct output (marked as carbonator) which is set very high 145psi and two which are adjustable. Because it looks designed for dispensing fizzy drinks I guess the tanks have a high head pressure to carbonate off the regulator plus two lines for dispense. You wouldn't want to run three kegs off this put it that way, the carbonator line is far too high, potentially would explode a corny, would be almost worthless and probably want blocking off or you could buy and fit an additional secondary to it.
On the topic of seeing how much gas is left primary regulator gauges don't tend to be a lot of use. You might think of it as a fuel gauge, but the majority of tanks are top dispensing vapour release. Imagine a tank almost completely full of liquid co2 which remains liquid at ambient temperature because of the immense pressure inside the cylinder, just above this is a vapour layer which is all can boil off and evaporate at these pressures. As the pressure drops (release of gas) the liquid co2 boils off more vapour to equalise the pressure (some of the liquid evaporates). The result is they show an almost constant pressure (gas/pressure equilibrium) until they are pretty much empty of liquid then they drop off to almost nothing rather rapidly. The best way to determine usage with something like a soda stream bottle is by weighing it?
I run all my kegs off a single duel stage regulator. It shows a (worthless) dial for primary pressure and has adjustment and dial for secondary pressure (output). A bunch of 3/8" speed fit and connector T's allow me to plumb in all my kegs and I use a non-return to prevent liquid coming back into the regulator (a risk when you hook very full kegs up in certain orders). Like if I've the bottle turned off and connect a keg which is filled past the gas post and holding pressure it will push beer into the gas lines and if higher than their pressure into other kegs. I tend to serve some beer before connecting up the gas post on new kegs to prevent this, or maintain crazy high line pressure when connecting up kegs which I later back off, but again you can sometimes push beer into other kegs on the same line through the gas lines through pressure differential without non-returns.
Doesn't this setup give me the same pressure on all kegs? Yes. Is this a problem? Not really. I naturally carbonate or force carbonate separate from the dispense system. Non-returns mean they don't 'talk' to each other and share carbonation. Typical temperature and dispense pressure creates an equilibrium of approx 2.3 volumes which means over time some get a bit fizzier, but not a lot because it is like ... 7C absorption without ridiculous differential is REALLY slow at those temperatures (weeks and weeks) and some get a bit flatter, but not much because the beer is already carbonated and again, the differential is very slight, only getting flatter faster as the keg is close to empty and there is more head space in relation to beer.
Think of it this way. A keg carbonated to 2.6 volumes will already be at a pressure of 16psi. I dispense at about 14. As beer leaves the keg no gas enters until it drops below 14 at which point gas maintains a constant 14. co2 will leave the beer to maintain a pressure of 16.
The conversion factor from volumes of CO2 to CO2 by weight (g/L) is 1.96. For example: 2.6 volumes x 1.96 = 5.09 g/l. To convert back to volumes, multiply by 0.51. My keg contains 101.8g of co2 when full of beer carbonated to 2.6 volumes. It can hold onto 90.16g at a head pressure of 13-14. I lose 0.0101395g per litre dispensed to equalise the head space which is 0.005171145 volumes per litre or 0.1034229 over the life of the keg.
I can live with that. I rarely brew anything really fizzy.
