Yeah, there's a lot to cover with the spacies mu's. I'll try not to go too far in depth, to put it simply there's a lot of tricks that work on either one that you should utilize. One extremely important thing is not to treat the two of them the same at all. Puff is an extremely mental-oriented character, you have to make them be scared to do anything punishable and control the match and the map. Constantly change your jump/attack patterns especially if you notice them trying to play defensive - they're stalling to try to read. Stay on their *** the entire match, don't give either spacie any room to run around and do what they want.
Fox is very quick and has an extremely deadly usmash and (uthrow+)uair, so you're usually going to die around 70-90%. Because of Fox's speed though, you can make reads pretty easily. Things like jumping at them like you're going to bair them, but then double jump and bair above their head and weave back. It either hits them as they jump or makes them scared to jump on your approaches. Also, WD oos is so good. WD back grabs are really good for gimp setups, but also wd oos fsmash can be very useful at times. You have to be creative, it's the only way to get in Fox's head. Try to think a step ahead, because Fox goes to the next step quicker than Puff.
Falco is a bit simpler. Stay above his shorthop (laser) range most of the time - after all, what's falco going to do when you're there? If they're playing campy just stay on their *** but don't go full on aggro, giving up your stage control and the potential hits you'll take approaching that way usually isn't worth it. Cornering a falco by the edge is exactly what you want to do. Bairs are a lot better in this mu, aim for the area right on front of and above their head. Falco can't really kill puff easily, just keep an eye out for utilts and dtilts at high percents - stay distant and safe. One of the biggest annoyances at first is dealing with dair spam, but once you get used to punishing it they have nothing left to turn to. (Falco's dair has very little horizontal coverage, just stay mobile and you can easily dodge and punish.) At this point, Falcos will have to start fullhoping and running around the stage looking for opportunities. If you just don't give them anything to work with, this mu isn't very hard.
And for both of these MUs, especially vs Fox,
weaving nairs are so good.
Don't try to go instinctive Hbox mode like a like of Puffs do; spamming bairs works on certain characters and in certain scenarios but it's more important to see what works best in the MU.
