Wow, finally someone ELSE references Crossfit

But you're misinformed 'no more than 21 reps' thing. There is no 'typical' Crossfit workout - rep counts can be as low as 3 or as high as 400. The mantra is: Routine is the enemy. Only constantly varying stimulus will force your body to keep adapting.
It seems there is one of these threads every week, and the same people post the same advice. I think i'm going to work up a 'Cliff Note's' version of what I usually post just to save time! All this is gleaned from crossfit.com and related sites. It's all time-tested stuff.
- 90% of weight loss (or weight gain) is diet. Clean up your diet before you do anything else.
- Keep workouts short and intense, for the most part. Your body responds much better to intense stimuli than moderate stimuli, plus it's more convenient and more exciting.
- Where applicable, use a stopwatch and keep track of your performance - the motivation effect is huge. Or write down how many reps you get in in a set amount of time. Or how far you swim etc etc. Write down everything you do and refer back to old workouts regularly. Seeing improvement on benchmarks will keep you going even when you're not seeing improvement in your waistline.
- If you're going to lift, you should lift heavy too. Strong people are more useful than weak ones, generally. Also, you're over 300lbs! You NEED to be strong if you're going to do any other form of exercise. You can't get much bigger than you already are so don't worry about lifting heavy.
High reps of low weight is fine as part of a varied approach, but just realize that it's a metabolic conditioning exercise, not a strength exercise. You need both metabolic conditioning AND strength to get where you want to be (acrobatics & parkour require crazy amounts of full-body strength on top of huge metabolic capacity). I'm pretty sure the main reason why your gym teachers want you to keep to a 'high reps, low weights' is because that's much safer for the untrained.
- Stay away from machines. All they do is remove the stabilization element from the work. Get some training in free weights and never look back. Body-weight exercises are incredibly useful as well - push-ups, pull-ups, squats, lunges, sit-ups, jumps...
- There are many, many more useful exercises than curls. Time spent doing isolation exercises is time when most of your body is doing nothing. Do exercises that recruit more muscle groups (like squats - you're on the right track there, just don't use a Smith machine, as above). Curls are for making your biceps look big, which is not one of your stated goals.
- 'Toning' is a myth, as thankfully some people have posted. Unless your body fat is already low enough to SEE the muscles under your skin, doing crunches is not going to make your gut look better. To see your abs, you need to lose fat, period. (Nothing wrong with doing sit-ups of course - just understand what it is that they're doing for you - strengthening your core).
- go to crossfit.com and read, read, read. Then watch the videos. It's all free. No-one is trying to sell you anything. Most of the info about exercise, diet and fitness there is hundreds of years old. Then try some of the workouts, scaled back to your current abilities/fitness level. If afterward, you don't feel more alive than you have in years, you get your money back!