Depends how fizzy you like it. You can easily go 150 to 200kpa on a cold keg. I usually set reg to 300 and shake for a few minutes, burp and serve at 100, that's at 5 degrees.
If you have beer on the same gas lines put a non return valve at the gas in of your soda keg so gas can't escape from there and overcarb your beer.
I recommend only making soda water and then flavour each drink as you pour it, gives you more variety and pure soda water.