Hiya! I think it's great that you're doing Pilates. I started about 4 or 5 months ago, used to do it 3 times a week, now I'm down to once a week because I do a lot of other exercise, too. In my experience, mastering the moves takes a lot of time. There are still some in the beginners' series that I can't do at all, and quite a few that I'm not doing with perfect form. I'm not very flexible, either, but I am seeing improvement just from the short time I've been doing it (I take a mat class and also work with a tape--the Ana Caban beginners' mat). So my advice to you would be to stick with it--it might seem time consuming to learn, but what the teachers say at the class I attend is that it's an ongoing process of improvement--for everyone, not just for beginners.

Three times a week seems fine--my teacher once mentioned that it's ok to practice every day, although I wouldn't go that far, especially if you're doing The Firm, too.

And yes, anywhere between 5 and 12 reps is the norm, depending on the exercise. I don't know that particular tape, but I'd just do it as the instructor is doing it.