MacBook – Why are two identical external monitors displaying at different resolutions

You will not be able to drive a Mac Mini to a monitor with a resolution of higher than 1920 x 1200 off of that HDMI port (different versions of HDMI have different capabilities vs. different versions of DP/mDP--the Mac version of HDMI is old). For 2560 resolution you must attempt to use the Thunderbolt ports to connect to the display mDP ports--in theory, although they are not identical, as the thunderbolt capabilities of data transfer are a superset of the mDP's, the thunderbolt port is backwards compatible with the mDP port and the Mac Mini will recognize the mDP monitors. Problems have been reported in practice in achieving full resolution for the mDP displays; there are also positive reports.

see: http://www.techadvisor.co.uk/feature/digital-home/displayport-vs-hdmi-3535257/ and: https://support.apple.com/en-us/HT204388

It's possible. You can run one monitor off the HDMI port and another 2 off of the Thunderbolt ports. You can get adapters for both of these.

There's more out there than this but here's one Apple sells.

http://www.apple.com/shop/product/MB570LL/B/mini-displayport-to-dvi-adapter

BTW: Thunderbolt ports can work Mini Display port adapters.