My daughter did the EXACT same things when she was 6.5 months old. She started sitting at 6 months, but hated tummy time, didn't roll over. She also loved being held up in a standing position and had those strong, stiff legs.
I really don't think you have anything to worry about. Aroung 7 months, my daughter started getting more comfortable on her stomach, and around 8 months, she finally started rolling over. After that, she started crawling around 9 months (but she still army crawls, instead of crawling more upright. I've done a lot of reading up on the subject, and the consensus is that all babys are different when it comes to crawling. They'll all do it at their own pace and in their own way.
For now, all you need to do is encourage that tummy time. I got down right on the carpet with her. But really, once she turned over, my daughter did most of her tummy time on her own in her crib. I would leave a few toys in there, and every morning I would realize that she's been up for at least 20 minutes quietly playing with her toys on her tummy.
But of course if you are concerned that there is a more serious condition, ask her pediatrition.