Its soo hard to edge guard in this MU but thats part of what makes it fun. I'm not a fan of fireball edgeguards against spacies UNLESS you have time to throw down a fireball and still cover a couple more recovery options. The fireball seems more and more like a lazy roll of the dice that I admittedly do way too often; I think you can almost always cover more options by not using the fireball but It's still decent to try it out against unsuspecting people and as a hard read.
I feel as though Mario has to go off stage when edgeguarding spacies, they just have too many options for you to cover if youre standing on stage, and they can often react to the option you cover on stage and then they will just choose a different one. In my experience I have the best luck going offstage but ALL of this post so far has been talking about spacies recovering when theyre above ledge height. If theyre below ledge height without a double jump then it gets far easier to cover their options even from just the ledge.