They fit exactly the same printers — the XL simply holds more ink. If you print often, 18XL works out cheaper per page and means fewer cartridge changes. For light use the standard 18 may be enough. You can run one of each in the same printer without any problem.

Epson names each cartridge family after a picture printed on the box — 18 is the Daisy range. It is only an identification aid for shoppers and has no bearing on the ink. Order by the number.

Genuine Epson stock for Expression Home printers of this age printers has largely disappeared from the shelves, and where it does turn up it is often old stock that has been sitting for years. Compatible cartridges are made new, which for a printer of this age is usually the more reliable option as well as the cheaper one.

No. The 18XL 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.