I like skinny jeans and wear them with thongs all the time. But personally I'm not fond of the underwear showing regardless of being a guy or a girl, that's a no for me. Unless is a lovely corset or bra under a sheer blouse - that I like a lot!
Always buy womens jeans, jeggings or skinny fit. They always fit fine, usually get them from NEXT as they do extra long lenghts in most of their jeans.