Depending on which font(s) you're planning on replacing, the requirements will be different. Of the fonts you can specify in the plugin settings, System_GUI_Font_Fixed_1 must be a fixed-width font - you'll get wonky results if it isn't. Areas_Label_Font is also a fixed-width font by default and as it's used to build the area label from a number of data fields that are meant to be specifically positioned relative to each other, you may have to experiment a bit if changing that font.
As for the "EuroScope" font itself, used in all the EuroScope-drawn track labels, menus and flight lists, the results will be hard to predict if the font is changed to be variable width. In addition to EuroScope itself, the font is very likely used in various plugins so changing that font from fixed width to variable width or changing its character height/width ratio significantly may cause issues when code expects a specific text string to fit exactly in a specific area.
