The "SMD" portion of the description is how many LED panels are on the fixture. 5 panels vs 18 panels.
The 18SMD should fit as it is the same length as the 5SMD according to the dimensions. However, it looks like the 18SMD might be mono-directional, meaning it will only put out light in one direction. That would be fine for the dome lights, but I think it could make the footwells look strange because of the orientation of the footwell receptacle. However, I do see that they are polar, meaning that they have to be inserted in a specific direction in order to work properly.
Edit: since they are 18SMD, that means they have panels on both sides. Should be good to go!
Thanks. I had no clue what the SMD was. Thanks for the great explanation.