You don't need to load the 'selector' core, almost every core has the option to change the FPGA core using the F12-menu (or pushing the middle button on the device itself) - there are some exceptions, i.e. the Atari 800, the PC Engine, the MSX and most arcade cores, see this spreadsheet: https://docs.google.com/spreadsheets/d/ ... edit#gid=0
I recommend to boot your system using a core.rbf default which has a VGA-standard compatible video output (640x480@60Hz).
Later you can choose other, less VGA compatible cores from the menu (i.e. PAL-standard: 720x576@50Hz), and switch the 15kHz/31kHz sync option pressing long the middle button on the device, if you have one of the latest firmware (or change the scandoubler_disable option in mist.ini: https://github.com/mist-devel/mist-board/wiki/DocIni
Important: in 15kHz mode the HSync and VSync signals are merged on VGA pin 13 to CSync (original HSync pin), and VGA pin 14 (the original VSync) gives an RGB switch signal, suitable for 'minimig VGA2SCART cable'. See the details here: https://github.com/mist-devel/mist-boar ... ScartCable
Perhaps the problem is that this is not what your Dell monitor expects in 15kHz mode (it needs probably separated HSync and VSync signals).
You can try a 'Sync-on-Green' cable instead: https://github.com/mist-devel/mist-boar ... PbPr_Cable