Can you use this example also with Olimex iCE40HX8K-EVB? Are there any changes in that case? Is an open source tool-chain also available for iCE40HX8K-EVB?
Yes, you can use it. If there are changes needed, they would be minimal, but I assume there are none. The same tool-chain will work for both iCE40HX?K-EVBs. Links are on the product pages.
Can you connect a PS/2 mouse to the PS/2 connector of Olimex iCE40-IO IO-modul VGA/PS2/IRDA? If you use Y-splitter cable for PS/2, can you connect both a keyboard & a mouse at the same time? Otherwise I think you should develop a new version of the board that can use a Y-split cable.
The PS/2 connector just connects the input device to the FPGA. What happens with then has to be programmed in the FPGA.
https://github.com/OLIMEX/iCE40HX1K-EVB/blob/master/demo/ice40-io-video/example_7.v shows how to use signals from a keyboard. You could just as well program correct responses to mouse movements. There's an example at opencores.org:
https://opencores.org/websvn,filedetails?repname=ps2&path=%2Fps2%2Fweb_uploads%2Fps2_mouse.v. Once you have both working, you can study how the splitter multiplexes the signals, and handle that on the FPGA too.
Good luck with your project!