I would add less than you think it is going to take. Can always add more if you need it but can't remove it without a partial drain. I have never used the liquid so I'm not sure about adding it. Read the directions on the label. But, if you use the granular, you can either put it in an old sock and hang it in front of a return jet or pour it into the skimmer and let it dissolve in your filter. If you do it the second way, don't backwash for up to a week to be sure to give it enough time to dissolve.

It won't hurt to ahead and add both cl and stabilizer tonight. But, if you decide to only do one, do the chlorine tonight.