The rod/slide works through friction. While you're moving the prosthetic, the rod or slide rubs against the clitoris. Proper positioning and size are crucial. The issue when you're pre-T is that your anatomy may not be large enough to come into contact with it at all. This could be further impacted by the choice of positioning during sex. You may have luck with it pre-T if you are naturally on the larger side, but I don't think most people are.
If you've already bought the prosthetic with the pleasure slide, I would just give it a shot and see. I've never used a freetom, but the functionality of it seems very similar in concept to the peecock's pleasure rod that I do have. If you don't have any luck with the pleasure slide due to size, I don't think you would have luck with the pleasure rod. The exact application is slightly different, but both are going to be impacted by the same factors.