Can I use any ink in any fountain pen?

Any fountain pen ink, nearly always yes — the cartridge fitting differs by brand, but bottled ink is universal once you have the right converter. The hard no: India ink, calligraphy/dip-pen ink, and anything with shellac. Those are for dip nibs and will cement a fountain pen feed permanently.

Within fountain pen inks, the care ladder runs dye (carefree) → iron-gall and pigment (clean on schedule) → shimmer (shake, write often, flush well).