LB, it’s totally fine to wear tennis shoes or any other kinds of shoes (open-toed, sling-backs, slits on the side, shoe-boots, you name it) with tights/thigh-highs/socks, as long as it is visible that you are wearing something to cover the leg that is exposed past your skirt. Nobody really cares. Many, many chareidi women wear tennis/athletic shoes, often for reasons of having suffered from varicose veins during multiple pregnancies and they need the support for their legs/feet, etc, whatever the reason. It can be just your style and you like that kind of shoe. Go for it.
I do want to point out something I see in this thread and was also mentioned in another thread. There is what is called a pony tail (known in EY as a koo-koo) and there is a braid (called a tzamah). Most Bais Yaakov girls wear their hair in a pony tail and that is totally tznius. All of the hair (except for some bangs if you like) is collected into a holder off the face. The braid (sometime 2) is worn more by the Yerushalmi/chassidish girls.
I am not sure where it says that you have to have your hair in a braid, I do recall this being mentioned in different thread but I do not have time to go reference it, but I hesitate to say that the large majority of chareidi girls are going around not tznius because their hair is merely in a pony tail and not in a braid.
Open hair is considered prost (I believe that is the word) and I personally consider long-haired shaitels in this category as well (nobody else in the entire world has to agree with me).