Need help to understand my electric panels and subpanels
You're going to have to start by mapping your house.  It helps to have a whole bunch of night lights, or any kind of lights, or loads of some sort that light up clearly when the power is on.  One for each receptacle in the house.  

I'm fond of making up names for them.  So for instance the Square D breaker box, I might call those Quill, Rocket, Gamora, Groot.  Then in the fuse block, call those Thor, Loki, Sif.    Then in the main one, use names of other Avengers not affiliated with the Guardians or Asgard. 

Starting with the subpanels, pull one breaker or fuse.  Then walk the house to see what dies.   Label them as you please, so you know what that outlet or hardwired device is powered from.    Repeat for each subpanel breaker or fuse. 

Next, go to the main panel (the StabLok?) and do the same thing.  Two of them will most likely shut off power to each subpanel, and you'll know that because everything connected with the subpanel will die.  Label that Asgard, Guardians or whatever grouping you are using.   All the other fuses will shut off some loads or one load.  Again mark the outlets/hardwired loads.  

Now you know what circuit every outlet and hardwired load is on.  

Next, read the number on the breaker or fuse.  That is your circuit limit for that circuit.  Figure 80% of that (12 amps for most of them) and do not exceed that. 

Generally, as far as the common 1500W heaters, you can put one of them per circuit, with nothing else except maybe a cell phone charger or at most a TV.  

To find out what loads draw, either read their labeling, or get a Kill-a-Watt and measure them (at maximum working load, e.g. Full-on FPS gaming, not editing a Word document).