Up to you what to use
Different people use different things... i don't think you can get away with topdressing SS alone if there's no other nutes it wouldn't be enough unless you left room to do topdressings every week or two... i would just run SS from the start if that's your end plan.
600's going to be stretching in a 5x5 space... you will have to raise the light substantially to get 5'x5' coverage which means you're not going to get the intensity you'd get from a smaller space. Also 5x5 for a 600w is 24w/ft2, which is less than half the ideal wattage per square foot. You're yields/bud size are not going to be as much as if you get it up to 50w/ft2 and keep the light down close to the canopy. You're gonna have to keep it at least a foot or two over them to get it to cover 5x5. I'm stretching it on mine covering 4'x4' and that's with a big ass hood designed for that kind of coverage. Just an fyi -- that's your biggest problem at the moment, not the soil/nutes, which is just whatever your preference is
that light is definitely fine for veg, just not optimal for that space and flower. But you don't have to go optimal either... i'm sure you'll get fine results depending on what your goals are... if you just want to grow some dank for yourself you should be fine. If you want to get max yields you've got an issue to address.