For plants to add/consider:
Dianthus deltoides
Jasione montana
Trifolium repens
Thymus polytrichus
If you can stand some that will get taller if you leave it unmown for a bit but will take mowing (just not flower unless you leave things to grown until, say June):
Geranium sanguineum
Silene vulgaris
Veronica chamaedrys
Dianthus gratianopolitanus