Lucina and Chrom sharing a slot is possible from a technical standpoint. Don't forget, the difference in height between the two trainers is bigger in Wii Fit than it is in Smash Bros, and they still are not equally as tall in this game.
From a personal standpoint, it's hard to say. I was going to go on about how they have different stances and it would really suck if Lucina took on Chrom's stance since her stance is so cool, and how Lucina's stance doesn't make sense for Chrom (it's intended for a shorter warrior)...
But you know what? I would just be dancing around the elephant in the room. Chrom is just a bad pick. A lot of his stylistic choices literally build upon what Ike started years ago.Between people saying he would be a ****ty addition to Smash Bros, all the R34 of him on the Internet, and how much a lot of Fire Emblem elitists hate the guy, I can't blame him for sighing at whatever he sees.