No. The XP-5100 uses the 502 range and the XP-5200 uses 503. The two printers look similar and sit in the same family, so it is an easy mistake. Check the number on the cartridge you are replacing before ordering.

They fit exactly the same printers — the XL simply holds more ink. If you print often, 503XL works out cheaper per page and means fewer cartridge changes. For light use the standard 503 may be enough. You can run one of each in the same printer without any problem.

Yes. The 503XL 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.

Epson printers show a message whenever a cartridge that isn't Epson-branded is fitted. It's a notification rather than a fault. Confirm the prompt on the printer screen or on your computer and printing continues as normal. It may reappear after the printer has been switched off and on again.