Germans are, and have always been, one of the main nationalities of the Foreign Legion, more so in the years after WW2 than at other times. This has natural explanations. The career options for a german professional soldier after WW2 were severely limited. For some German veterans, the legion may have been the only career available to them. Others may well have enlisted due to difficulties in adapting to civilian life.

As to Waffen SS, yes, there were members of the Waffen SS in the legion after WW2 and yes, they enlisted under pseudonyms (as did everyone else, it was required at the time). This does not however mean that wanted war criminals were able to escape to the legion. The french intelligence were well aware of this possibility and did keep an eye on the situation. To my knowledge, there are no known cases of Germans wanted for war crimes serving in the Foreign legion.

There is also a heavily romanticized myth (largely due to the book Devil's Guard) that Dien Bein Phu was "the last battle of the Waffen SS", this is of course absolute bollocks. The average legionnaire at Dien Bien Phu was 23 years old and as such was only ~14 when WW2 ended. Far to young to have been a member of anything but possibly the Hitler Youth.

The legion recruitment policy is that no one nationality can ever account for more than 25% of the total number, even if we assume zealous recruiters willing to accept germans other nationalities it's unlikely that their number ever exceeded 35% or so.

March Or Die: A New History of the French Foreign Legion - Tony Geraghty
The French Foreign Legion: Complete History of The Legendary Fighting Force. - Douglas Porch
The French Foreign Legion: An Illustrated History. - Jean-Denis Lepage
Street Without Joy: The French Debacle in Indochina - Bernard Fall