Hi Sara,
I agree that you shouldn't get discouraged comparing yourself to others. And, there is more than one way to skin a cat. (Or, in this case, lose fat.)
This being said, some ways of skinning cats are safer and more efficient than others. Your comment about being too short for the machines, for example, is a good reason not to emphasize machines so much in your workouts. Sooner or later, that extra stress on your joints from lifting with a machine that isn't designed for someone your height is going to catch up to you.
Similarly, the advice not to use the ab/adductor machine is based on the knowledge that you get more bang for your exercise buck with compound movements. (These are the movements that exercise leg, glute, and stabilizer muscles in lower back and abs at once, rather than one teeny tiny muscle at a time.) Sure, the ab/ad machine might not do any harm and it's certainly better than nothing, but it also isn't the most efficient way to spend your weight lifting time. Considering how many people get discouraged if they don't see results, "little" things like exercise choice can mean the difference between success and quitting.
If you're confused, start by reading. But don't just read here. I know I sound like a broken record, but the best source of information on the web, bar none, for well-researched and sales-free advice on weight lifting and fitness that's geared toward "average" women is Krista Scott Dixon's site,
www.stumptuous.com (click on Iron, and start with the "starting" and "training" pages). If you want a second opinion, take a look at some of the books she reviews and recommends. As with anything else, the more you know, the more confident you are, and so forth.
As for personal trainers or gym trainers, there are good ones and bad ones. I've seen a lot of trainers in gyms who don't know diddly squat about training, or who spout off on "expert" advice about what women can or can't do based on stereotypes. Unless you arm yourself with a good foundational knowledge, it can be hard to tell the difference between a good trainer and a bad one. You don't have to get a degree in exercise science: a couple of hours on Krista's site will likely be enough.
Something I read on a weightlifting newsgroup 10 years ago stuck with me: more is not better, better is better!
Kim