First things first: What brand are your shifters and derailleurs? 7-speed is probably an indexed setup, so shifters and derailleurs need to be designed to work together.

If you've got a Shimano drivetrain, 7-speed road and mountain bike rear derailleurs are interchangeable. For that matter, Shimano rear derailleurs are generally interchangeable up through 9-speed since they all use the same cable pull ratio.

What kind of gearing do you have on your bike? Not just how many 'speeds', but the size of the smallest and biggest cogs on the rear wheel and same for the chainrings on the crankset (in number of teeth). With those numbers, we can do a little math to tell you definitively what will and won't work. A long cage derailleur will work with wide gearing ranges, but you may not need that longer cage.