There is a lot of confusion in Astrology over the term rulership - even amongst professional Astrologers. There's a least two distinct ways in which the term is used. Firstly as expressing a strong affinity between planet and 'thing' - thus, for example, Venus is said to rule dancing, love, joy and Green Jaspar (and loads of other things).
The second distinct usage is in the sign rulership and here the originating principle is not affinity but seasonal. The Sun rules the hotest month/sign of the year (Northern Hemisphere) - Leo. The coldest month/sign of the year - Aquarius iss ruled by Saturn because it was the visible planet furthest from the Sun (and thus the coldest). The Moon, as the other Astrologial light, ruled Cancer and the Summer Solstice. The Winter Solstice and Capricorn was ruled by Saturn. Working back from the cold signs of Capricorn /Aquarius moving in both directions around the circle of the signs, Jupiter ruled Pisces and Sagittarius, Mars ruled Scorpio and Aries, Venus ruled Libra and Taurus and Mercury rules Gemini and Virgo.
Now on the first interpretation of rulership there is room for argument and discussion - Tradtional writers clearly differ over the planet that rules owls, with Venus, the Moon (for owls out at night), Mars and Saturn all getting a mention in this role. It's clear that the rulership depends on how you see the nature of an owl. At a modern level it's quite possible to argue whether Mercury or Uranus is a better (stronger affinity) ruler for Computers.
However on the sign rulerships if you say that Pluto should rule Scorpio, then there is a problem. If this is done on the basis of affinity, it contradicts the system based on seasons and therefore invalidates Mars' rulership of Aries and indeed all the other rulerships. So my short answer is that Venus rules Taurus and Mercury rules Virgo and this is the case because it is based on the system of sign rulerships that has been handed down to us.
To further add fuel to the fire, I think I'm correct in stating that both Vulcan and Pan Horus are 'hypothetical' planets - their existance is not confirmed. Personally I don't think Taurueans are ruled by a probably non-existant planet
That being said threre are clearly Astrologers who do use hypothetical planets and I am not suggesting that this approach is invalid - though I do question whether such planets can be usefully used as rulers, especially when no confirmed ephemerides exist for them.