Couple thoughts off the top of my head:

You seem to know already that you're going to encounter some issues with that many different plants on the same res. Your ppm is high for some of those plants especially at the stages of growth some seem to be. That high for a young plant like your brussel sprouts can burn them up or completely lock them out from getting anything at all...water or nutes. That may be what's happening with that plant. Another thing I see is that peat cube could be a problem. When you flood your buckets is that cube getting wet and staying wet? Hard to tell from the photo but those cubes staying saturated can cause dampening off and if your roots of that plant aren't developed well outside the cube they'll stay saturated and that's not good.

The cucumber looks like a fungus to me, not burn from the lights.

Squash does look like a mg deficiency or several other things. How often do you do a water change? Are you tracking your ppm between changes to see what's going on?

That makes sense. I've only been growing for two and half weeks, but I changed the first drum of water after 2 weeks. And everything seemed to fine, except the fungusy thing on the cukes, I'm not sure what to do about that since it's only 1 plant.

PPM stayed around the same throughout the time period. It would slowly go up, but go back down as I added more water (with adjusted pH).

I'm worried about the peat cubes as well. The guy at my grow store told me to just toss them in there once they are germinated and big enough. I'm not sure how to rectify the problem as there are roots coming out all over the place...

What I did tonight was to just flush the plants all with fresh water. Dump to solution, and fill it up with ph-balanced water, which I thought I'd do for a day before dumping and making a new nutrient solution (with half the nutrients as before). Sound like a good plan? I'm still not sure what to do about the cuke fungus...it's odd that it's only on one plant, and it's a pretty contained environment indoors.

Unfortunately diagnosing plant issues whether they're fungus or virus or deficiency/toxicity is difficult since many symptoms of different issues are identical or very similar. The cucumber looks like a fungus to me but also possibly potassium or manganese deficiency but considering the size of your res and amount of buckets you're running coupled with the fact that your ppm is remaining stable it's doubtful it's a deficiency. The other option to consider is with your ppm level for some younger plants excess potassium can interfere with the uptake of a bunch of other nutrients. So what may look like a deficiency is not really that it's not there for the plant to use but that the plant can't uptake it since, in this case possibly, it's being blocked by an excess amount of potassium. You said you're using MaxiGro which is what I use as well for most plants and is a really good mix for vegetative growth but at 1400 ppm it may have been too much K for the cucumber. Some plants are picky.

Most people will do what you did which is just put the plant and the peat cube right in the bucket. And there is nothing wrong with that except for if the cubes get saturated and stay that way they can get mold, fungus, or algae growth which can hurt the plant. Using them is not the issue it's just you need to make sure they aren't getting saturated and control that with water level. I put my plants directly into the hydroton after germination them in a soilless mix. I don't want anything in my buckets that doesn't need to be there. The dampening off that you can get from it usually is just a seedling issue and once plants get bigger it's not a problem. I would just make sure those cubes are staying saturated.

What you are doing sounds like a good plan. Flush it out. Start with a weaker mix and go from there. At 1400 ppm, depending on conversion factor, it's either 2 or 2.8 CF. I run my full grown fruiting tomato plant, very nute hungry plants, at 2.6. So that poor brussel spout seedling was getting blasted. Some young plants can handle high levels but other not so much. You're probably going to have to work your way up and find a level that feeds your hungry plants on the lighter side and your not so hungry plant on the heavier side. You're not going to be able to be optimum for all of them.

One more thing. If in 2 weeks ppm was staying stable given the same water volume it's too much in my opinion. Everyone has there own way of doing it but I like to see my EC levels dropping on a daily basis. I top up my buckets to a consistent water level and check EC daily. As you noticed when you're water level dropped your PPM went up. Add more water and it goes back down to probably around where it started which means you're plants are using much nutes in relation to the concentration in your res. I like to see a daily drop in EC. Too low a concentration of nutes can cause uptake issues as well as too high a concentration. I like to find that level where I know nutes and water are getting used daily which means I am more in that sweet spot. Hope that makes sense.

Thanks, that's very good advice. And I think I know what happened now. Like I mentioned before, when I was flushing and refilling the reservoir on Monday, I grabbed phDown instead of up, and it took me awhile to figure out why my pH wasn't balancing before I realized I grabbed the wrong bottle. So I had to use a lot of phUp to compensate. Which probably gave WAY too much potassium. Because they were flourishing prior (minus the cucumber's fungus/disease/whatever it is). I just hope I can salvage the plants. If not, it's only time wasted, and lesson learned.

1400ppm seems REALLY high, especially for babies. I have a 3mo old jalapeño plant in a 3.5g bubbler using normal GH and using just above the recommended nutes my ppms are only near 200 and this thing puts off fruit like crazy, even without direct sunlight. Either my notes are really, really weak or yours are really, really high, or both. Or my ppm meter is broken.