Is it permissible to use fractional aspect ratio W/L for MOSFETs?
Absolutely, it is done all the time in practice, especially when you want a device in triode.  I use it regularly in bias circuits or comparator preamps with triode loads.  From a practical standpoint, if you want to use a really long device, you shouldn't have the width at minimum but it is usually better to have it closer to 2x or 3x minimum.