Hello Jasol,
A good general rule of thumb for nutrient reservoir size goes like this;

Small plants like lettuce or strawberry's etc., 1/2 gallon per plant minimum.

Medium size plants like bush peppers, herbs etc., 1 to 1-1/2 gallons per plant minimum.

Large plants like tomato's or melons etc., 2.5 gallons per plant minimum.

I have never grown palms, but going by your maximum size of 20 cm (I guess about 8 inches), they would fall in the category of small plants to me. So 1/2 x96= 48. If I'm not mistaken those bato buckets hold about 1 gallon of water in the bottom each, that would be 11 gallons right there (11 bato buckets). So 2-15 gallon reservoirs (30 gallons) and another 11 gallons in the buckets would be about 41 gallons total (more or less depending on how much is actually kept in the buckets). You would most likely be OK with that, especially if you kept the water level topped off, and changed the nutrient solution at least twice a month (as they bigger).

Though it's always been my experience that a larger reservoir is always better. The larger water volume reduces nutrient fluctuation/concentration as the plants drink the water, and before you can top it back up with fresh plain water (thus back to it's original water level). Also with larger water volumes the nutrient solution dosen't need to be changed as often. I try to double the minimum water volume recommendations myself. But a lot has to do with the environment as well. In a hot dry climate (like mine) the plants will drink up a lot more water than they would in cooler humid environments. So temp and humidity will drastically affect how much water the plants drink daily.

Each grower has different conditions and environments they grow in, so each grower needs to do some trial and error to find out what works the best for them. But Hopefully I was able to give you a good starting point. I would love to see some pictures of your operation (both soil and hydro).

P.S.
You probably already know this, but small seedlings don't need a fill strength nutrient solution. So starting out with a low strength solution, and increasing it as they get bigger is better for them.

And that was exactly the kind of answer I was looking for.

Thanks HH. Will get some pics on here once I get everything up and firing - given that it's summer in Oz, the majority of my traditional palm stock has already gone. Still have around 550 left - a mixed selection of Foxtails, Christmas, Bottles, MacArthur and Alexandra.

Alot is going to boil down to how thirsty the little guys get. If I run on your experience with the lettuce/strawberries, then I should be able to get away with a fortnightly flush for the first four weeks while they are still in their infancy. After that will depend on whether they go ballistic or continue to cruise along under their own steam.

I honestly don't know what to expect - there doesn't seem to be much reported on the net from people who have attempted this with palms before. At the very least, it'll make for an entertaining distraction.