Why the list is only ever four names long
There are twelve signs and only two separations that classical practice files as comfortable: a third of the circle and a sixth of it. From Leo, a third of the circle in either direction lands on Sagittarius and Aries; a sixth lands on Libra and Gemini. That exhausts the possibilities, which is why every version of this list, in every magazine, contains the same four names.
The brevity is a clue about the method. A rule that produces identical output for every Leo alive is not describing Leos.
The Sun as ruler, and the odd consequence
Leo is the one sign ruled by the Sun itself, and this creates a genuine peculiarity: for Leo alone, the sun sign and the ruling planet are the same body. Every other sign sends you elsewhere in the chart to find its governor, and Leo does not.
That is precisely why sun-sign writing flatters Leo and misleads about it. The popular caricature, warmth arranged around a need for acknowledgement, is at least on the right subject. But the Sun in a chart describes purpose and vitality, not appetite or attachment, and those are Mars and Venus. A Leo with Venus in Virgo shows fondness by fixing your accounts, and no amount of solar description will predict that.
Fixed fire against the four candidates
Leo is fixed, belonging to the middle of a season, which is where seasons are least inclined to change. Set against mutable Sagittarius the tradition expects enthusiasm that Sagittarius will scatter and Leo will insist on finishing. Against cardinal Aries it expects two people who both want to go first. Against Libra and Gemini it expects an audience that talks back, which is generally read as agreeable.
All four readings are plausible as sketches and none of them is testable. They describe how the categories interact, not how two people did.
What a proper comparison examines
A sun sign is one entry in a chart with dozens. The comparison with content reads the Moon in both charts for what each person privately needs, Venus for how warmth is offered and how it is recognised, Mars for the temper of a disagreement, and the ascendant for what each person presents on first meeting. It then measures the aspects that run from one chart to the other, which is the technique called synastry. The synastry calculator works from two sets of birth data.
What it will not do is identify a soulmate, because astrology has never contained the concept. There is no placement for it and the older authorities do not attempt one.
A more informative half hour
Choose the sign you are actually thinking about and read that combination on its own terms, listed or not. Leo and Capricorn is unglamorous and considerably more instructive than Leo and Aries, because it has to explain a real difference rather than assert an easy one.
If both birth times exist, put the two charts together. It answers a narrower question than the one people arrive with, honestly.
Questions people ask about this
Which signs are traditionally matched with Leo?
Sagittarius and Aries in fire, Libra and Gemini in air. The four fall out of the two angles classical practice reads as easy.
Can a chart identify a soulmate?
No. The idea has no astrological placement or aspect behind it and does not appear in the older sources.
What rules Leo?
The Sun, which is unusual: for Leo the sun sign and the ruling body are one and the same.
What should be read instead of two sun signs?
The Moon, Venus, Mars and the ascendant in both charts, then the aspects between them.