Mixing treatments always put you in a type of experimental situation anyway since drugs are seldom ever tested together anyway.
When I did my original research for my wife back in 2004, LDN was little known in the UK so most of the people I made contact with were in the US and they had all started on LDN in addition to the licensed drugs. Most but not all had then dropped the licensed drugs when they found out LDN was more effective at stopping disease progress.
Those I did speak to that had kept on the licensed drugs in addition to LDN did not seem to be having any problems.
In 2004 the official story was that LDN could not be mixed with the licensed drugs, the official story now is that it can be. Not sure either statement was based on any evidence however.
All I would suggest is that if you do decide to try LDN in addition to rebif you keep careful notes on results. LDN can be very individually dose critical anyway so you really do need to keep note of results anyway to insure you have some basis to decide what dose is right for you anyway.