They fit exactly the same printers — the XL simply holds more ink. If you print often, 16XL works out cheaper per page and means fewer cartridge changes. For light use the standard 16 may be enough. You can run one of each in the same printer without any problem.

No. Epson gives each cartridge family a picture name to help people identify it on the shelf, and 16 is the Pen and Crossword range. It tells you nothing about the ink itself — the number is what matters when ordering.

Yes. The 16XL 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.

Yes. Refilling is the cheapest way to keep a WF-2510WF or WF-2630WF running, and it keeps empty cartridges out of the bin. There are however some additional costs such as purchasing a chip resetter to reset the chip on the cartridge.