The RIN and MRIN numbers you see in Legacy are actually a number added by the database software as you enter information. All genealogy database software automatically assigns a number (the next sequence number) as a new record is added, however Legacy allows that software generated number to be displayed.
FTM has a reference numbering system (see Tools > Properties > Reference tab on the Options dialog) you can initiate. The reference number is turned off automatically, so you would have to turn the system on if you desire to add reference number.
Since I do not use the system, might I suggest you read the specifics on using the reference numbering (Help and the two FTM books perhaps) and do some testing to see whether this system works for you.