Should I be worried that my baby scoots around on her bottom but is not trying to crawl?

No. In the field of developmental pediatrics, scooting and rolling around are all equivalent to crawling. Only about 25% of babies crawl on all fours.
No. Some babies scoot around. Most babies sit at 6 months and crawl at about 8-9 months. They stand up around 10-12 months. At each regular check-up, the doctor can follow the baby's progress to be sure progress is within the normal time-frames. The doctor can examine the baby to see if there is any unusual reason for scooting around instead of crawling. There usually is not a problem.
No. Babies army crawl, they accidentally crawl backwards and get mad, they crawl while dragging one foot behind them. All these get parents worried, and they are all innocent if done around the 9 to 12 mo time. After 12 mo, most children should be crawling. It's all based on necessity. If your child can scoot quickly, get to things, and be happy, then they will do that. They will crawl when ready.