Shimano RD TX55 Tourney TX 6/7 Speed Compatibility
Edit: First question, why did you replace the derailleur? 

Rear derailleurs don't really care about # of speeds - its the shifters which change the cable pull to move them around (so mixing shimano and sram can cause problems due to different amounts of cable pull). By RD not aligned with gears, do you mean the hanger / cage are bent? If so, go to your LBS and get them to unbend it / replace the hanger. If the rear derailleur is in good condition, you just need to adjust the cable (assuming its in good condition) to be taut when in the smallest cog (I lightly use pliers for this - make sure that the cable is actually being held in place) and then use the adjustment screws.

Have you adjusted your high low limit stops and indexing adjustment and btension?
See http://sheldonbrown.com/derailer-adjustment.html for an overview (or one of the various youtube videos on doing this)
http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

Also see the thread here: What are the most common causes for hysteresis in the rear derailler?