The Aquarius (ascendant or dominant as Sun is mainly the ego) if they have more Saturn in them I more shy reserved and has kind of a cold rational intellectualism a scientific mind and they conform a lot to Social things this is written about on several websites. I am Aquarius rising and I did a couple readings on friends who had this.
Honestly if i didn't have Saturn conjunct it (my ruler) in my chart I would have gone out and done all sorts of crazy stuff just to rebel when I was younger and constantly gotten in trouble. Even now I have ideas and philosophies that shock people. I get into debates and stuff in a fun way but am always too blunt and I don't really fit in anywhere fully that is Uranus sudden and explosive. So Saturn in my chart is a good thing cause it moderates that a little.
This ascendant is tiring though I must admit cause it's a little hard to control it's sudden energy and I get anxiety attacks and overstimulation too much.
Anyways about that I think it's the balancing principal the thing that keeps the sign scientific and rational and detached. I believe it's the Day ruler (Saturn)
No so far as Jupiter with Pisces I have no idea as I haven't really experienced that sign much. Someone else will have to answer but I am sure since the other two rule the sign in an obvious way that probably does too.
With Mars in Scorpio that is the feminine side of Mars obviously a combat of a different kind than Aries. I actually kind of like that placement and am facinated by it. I don't have it though. It still has a war like side and nature like Aries but is more on the Emotional and spiritual level. I think this placement would be an asset to any Satanic soul. Intuitively it can sense and read things and sting back in the most hard way. I think Mars rules this too as it's insanely powerful in that sign. I almost thought I was going to die physically a couple of years back and that feeling lasted for hours after I did too many mantras for Mars so be careful using that energy if you decide to.
Anyways that's my two sense someone else will have to explain Pisces I mainly wanted to add in my thoughts about Aquarius since I have that as Ascendant so I think I feel and understand it's energy.
Anyways I don't know what decided what planets rule what. If there was a 13th sign there is not anymore I don't think cause I have planet past around 15 degrees Scorpio and I am not fitting all that new stuff with it. I think it had to do with Phaeton the planet that was destroyed if it existed.