Guidelines in Glyphs Mini

Is it possible in Glyphs Mini to create horizontal, vertical or diagonal guidelines?

Yes. Available from the context menu.

There are no special horizontal/vertical or diagonal guides. Just guides with an angel. 0° means horizontal, 90° means vertical and every other angle would mean diagonal.

Thank you Tosche, I’ll check it out.

Thank you Georg, I’ll try that.

Do you mean drawing a guideline with the pen tool? I would like it to stand out in a different colour from the grey grid spacing and subdivision lines.

There are no free form guides. Only the background layer and the blue (local) and red (global) guides.