I dont consider myself uninformed, but i think i hot got this plant figured out backwards.
So, rule is, if its rubbery , its dry, if its mushy, its rot. Mine was rubbery , i watered it, and it was hard. This was, 3 weeks ago. Now its rubbery again, so i watered it again. Am i mistaken about its watering regime?

It's helpful to divide the year into seasons for this plant. The dormant season (when the plant has no leaves during the winter) is when you want to water it like a succulent, and let the soil dry out. Maybe the rubbery rule would work there. The growth season (when the plant is leafy through the rest of the year) is when you want to water it like a tropical plant, and increase the frequency. Especially in summer. During times of active growth, these plants enjoy much more water than your average succulent.

Here in our mild climate I give my Pachypodiums lots of sun and water them twice a week when they are in leaf. Right now that is how I'm watering all the lamereis. That is probably underwatering but I can get away with it and the plants are compact. It's important to use a rocky mix (say 50% pumice or perlite) and provide strong light.

The interval between flowers for the last 3 years on this plant tells you how much it grows in a year.

I dont live in ideal conditions, but im pretty sure that most plants accelerate as they age(of course , there are exceptions).
Yeah, ill move the watering regime twice a month for now, Maybe more in the summer.

Each location demands its own adjustments, so keep an eye on the plant and see how it responds. I used to water my plants less often, and the difference now is that they keep more leaves for longer. If you compare the picture above to some of the ones in the database from Florida gardens, you will see it looks sort of stunted. That is typical for Pachypodiums in the Californias where we get zero rain in the summer.

As for age related differences, I would think that younger plants require more frequent water than older ones, all things equal. They do better when the soil doesn't stay dry for too long. I water my baby seedlings twice a week year round for the first couple of years (and after the first year they are all out in the sun). Once they go in the ground, they do well with much less frequent water. Maybe once a week for the first year or so, then much less.

Your situation will be different if the plant is indoors, I'm just trying to share my experience so you can translate it to your windowsill.

Im well aware that seedlings need more water. That being said, mine is more qualified as a sapling(because its not old enough to have a wooden bark).I water these seedling 3 times a month(and dont think they need more than that). Save for the pachycereus box, have no idea as to what they are though,

Looking good. Just out of curiosity, what does your Pachypodium look like?

Here are 2 year old seedlings from the fruiting plant pictured above, in various stages of leafing out for the season. You can tell by the density of the spines when I moved them from their previous protected location to their current full sun location about a year ago.

My euporbiad/apocyanae collection. Old photo.