Are they all clones except for the lower right? It looks like they haven't recovered from the cloning process yet. The new growth doesn't look good and that's your real indicator man. For whatever reason your roots aren't working.
Usually roots stop working due to pH issues, which are caused by aggressive feeding.
To answer your question, a liter or quart of water should be good enough for those until the plants get about two feet tall, assuming you're using 3 gallon pots and assuming you water in small increments. Once they get bigger try a half gallon.
If I walked into my grow room one day and found my plants like this I would test the soil for moister and pH. I say test for moister because roots need oxygen to metabolize so if they sit in wet soil they can only pull nutes for so long before they run out of energy. If you over water, the roots aren't functioning most of the grow week so they can't keep up. Adding fertilizers changes the pH of the soil. Since you're growing in pro mix, which I think is a soilless media, you don't want to worry about microbes and humates and all that fun stuff, but you also don't have much of a buffering capacity so you have to keep an eye on your pH.
I don't think you need to be nuting them very much right now. Remember, their roots haven't spread into the soil very much yet so most of what you feed them just sits there until you flush it out. But once they are about a foot and a half tall give them a single dose of some organic veg fertilizer. There isn't really anything special about veg fertilizers except for the labels so look for something that has more N than K or P, and about as much K as P. pH your watering solution, water twice a week with a half gallon max, and be patient!
As far as schedules go, you can either feed lightly every watering or feed moderately every other watering. Just don't be the guy who feeds the maximum dose every time, doesn't water his plants enough for run off, then wonders who he has a nute build up a week before flower!
