Um....why would you worry that it's too late? How bad do they look?
And yes, you
could use that fert if had to, as long as you're careful to not give them too much. But, you'd be better off to pick some other fert with more nitrogen in it, even if it's the cheap stuff. That stuff you have would be better for bloom, because it contains high amounts of phosphorus, which is recommended for flowering. Nitrogen is the most important nute for vegging.