The film holders are the same - externally.
Both film sizes are about 2mm smaller than "nominal", which makes 5x7" 12.5x17.6cm. Funnily the 13x18cm film ends up as 12.8x17.8cm, or almost exactly 5" by 7"...
Taking the tolerances into account, there are 13x18 films which will fit in a 5x7" holder. But there are many more that won't fit. Conversely there are 5x7" films that will stay in place in a 13x18 holder - but most will be in danger of dropping out. The sound of a film landing in the bellows is almost as awful as the scrunchy sound you get when you try to insert the dark slide and the film has bowed out of the holder...