That's not true, no. You won't necessarily gain your weight back in double if you lose it to rapidly. A month and a half is plenty of time to lose 20 pounds and keep it off.

However, first of all, 1000 calories is not enough to eat under any circumstances. 1500 minimum, especially if you're working out. Secondly, how much protein are you getting? You want to make sure you're getting enough protein, otherwise your body will resist burning fat. Thirdly, you can't lose fat and build muscle at the same time and get significant results in either under natural circumstances, and even if you could you would be gaining weight, not losing it. You have to consume more than you burn if you want to build muscle, and muscle weighs more than fat.

If you need to drop a weight class by october then building muscle is not the way to go. Lift just enough to maintain, then cardio, cardio, cardio, along with a diet to make your metabolism work for you and you should reach your goal pretty easily. I'd actually try and accelerate it a bit for wrestling, lose about five pounds more than you want to lose in about two weeks less, then put on another five pounds of muscle before you weigh in. That's just what I would do though.