They fit exactly the same printers, the XL simply holds more ink. If you print often, 27XL works out cheaper per page and means fewer cartridge changes. For light use the standard 27 may be enough. You can run one of each in the same printer without any problem.

Epson also produced a 27XXL black for some machines in this family, holding more ink again than the XL. The colours remain XL. If you print a lot of plain text it is worth checking whether your model accepts the XXL black before ordering.

No. The 27XL range uses separate cartridges for each colour, so you only replace the one that has actually run out. A multipack normally works out cheaper than buying four singles, so it's worth a full set if you print regularly in colour.

If the printer reports that a cartridge can't be recognised, take it out and wipe the small copper-coloured chip gently with a dry lint-free cloth, then refit it firmly until it clicks. A poor contact on the chip is the usual cause. If it persists, switch the printer off at the mains for a minute before trying again.