The difference between download managers and download accelerators is that download managers manage downloads, they basically allow you to pause and resume them. Download accelerators on the other hand optimise your connection by changing generic connection settings on your os so that you get maximum benifit of your connection.

Also regarding software tweaks, they are neither illegal, nor will you get in trouble with the ISP's for using them unless you try to hack their servers or you try to steal your neighbours bandwidth.