I am planning on doing STS strictly as planned. I have never limited my cardio that much before, but you know what - I'm not that happy with my body right now - so perhaps limiting my cardio will be a good thing!! Also, if I find that I miss my cardio, I can always use STS in a 'different' way next time, but I'm gonna do the entire series as planned first, and see what happens to my bod. I'm so excited!!!
As for weight loads - I'm almost certain that somebody who can't lift more than 3 lb weights would benefit from doing STS!!
Also, I don't think 15 lbs is anything to be ashamed of - that's a lot of weight! That said, I bet you'll be able to lift even more than 15 lbs if you give STS a try. I can't wait to be able to lift heavier!!
In testing my 1RM, I was truly surprised to find that I've been lifting a lot lighter than I could/should be.
To find your 1RM - simply pick up a dumbbell and do something like a triceps kickback or a biceps curl. Go for 10 reps. If you can make it to 11 reps, try a heavier weight next time. If you can still keep going after 10 reps, try a heavier weight next time, etc. Once you find a weight that allows you to hit 10 reps that won't let you get to 11 - you've found your 1RM. And since the DVDs haven't even filmed yet, you still have lots of time to find your 1RM.
HTH!