If ferrous ions are so unstable, why do many ferrous minerals exist in nature? That doesn't really make sense.
Another thing, do all ferric salts dissolve iron to form ferrous salts? Is that a rule?
Not all ferrous salts are unstable. It is a matter of combination of anion and cation and even more likely, a matter of ligands.
For many metals, there is not a single preferred oxidation state. The preferred oxidation state strongly depends on ligands. E.g. cobalt with aqua ligands is stable in the +2 oxidation state, in the +3 oxidation state it is very strongly oxidizing, even water is oxidized by that stuff.
However, with ammonia ligands or cyanide ligands, cobalt in the +2 oxidation state is strongly reducing and in fact, it is impossible to keep a cobalt salt in the +2 oxidation state pure when these ligands are present. It simply sucks oxygen from the air, very eager to go to the +3 oxidation state.
With iron I expect the same holds.
Another reason, why many minerals of iron are in the +2 oxidation state is the absence of oxygen at the place where the minerals were formed. E.g. sulphur and iron give rise to pyrite (FeS2), with iron in the +2 oxidation state and S2(2-) being the disulfide ion. Many of these +2 oxidation state minerals, however, are oxidized to the +3 state, if exposed to air, especially, when combined with moisture.