horizontal comboing is important for light/floaty characters - like jpuff and kirby. not sure if jab-fair is legit, but ftilt -> fair is, as is dash attack -> fair, or reverse bair -> fair.
i think you're one of the only people to switch from pika to yoshi
jab fair is legit sometimes but not that often, you need some specific %s I think. I use it sometimes on like Mario or something, if I like, dash attack so that I'm standing on the very edge, and opponent is kind of inside me so that he's not far enough for run off fair, and ftilt feels kind of awkward or too slow and I'll probably run off fair on accident if I try to ftilt anyway, since I'm right at the edge. Yeah.
I kind of switched Pika to Yoshi
Watch yoshi combo videos and try to recreate them. Usually horizontal combos are done on characters like jigglypuff, Kirby, Mario bros., samus, and Ness. So any combo you see done on them would be good practice
I don't think they're too common vs Ness because he's not as floaty (he's more like Pikachu), though it kind of depends on what you mean. You can still do shorter stuff like fair nair fair or dash attack ftilt fair and stuff, I just mean like, you usually don't see a combination of several bair/nair/ftilt/dash attack on him like you might vs the others, those horizontal moves only happen once or twice in my experience.
Anyway I wouldn't know too much about what to practice because I think I mostly learned this stuff from playing against humans. I might be wrong about some this actually since I have a hard time imagining Smash stuff sometimes compared to actually playing it. But if you want something that works at 0% you could try DJC bair -> DJC nair -> dash attack -> ftilt -> full hop fair on Jigglypuff. All 5 hits connect I just checked. That's one I guess. A lot of this stuff vs the other chars often starts more at like 20% up.
I think combo stuff is hard to answer at times because this game is quite situation-specific and reality is more complicated than the answer people expect. I think it might be better to learn what moves generally chains together instead of memorizing training mode zero to deaths. You can then kind of put together a freestyle combo, that works, in most situations, if you know these things.