Yukikaze counters all attacks, but not throws. I wouldn't rely on it though. 4 stars is a lot.
Hakumen isn't really a mix up character. All of his attacks are slow and cumbersome. You can still do mixups, indirectly, by using frame traps, which is the best way to get your opponent confused with Hakumen. Basically you can link your moves together when they're blocking in such a way that there's just enough of a gap for the opponent to think they have a chance to use an attack, but the gap is small enough that they won't have time to actually land it. Instead they'll end up not blocking and get hit, letting you do your combo. You can also mix in throws, and the ocassional overhead; Tsubaki is very strong. Basically Hakumen's best mixup is to make your opponent think it's safe to throw out a move and then hit them over the head because they were wrong. You have to learn where the gaps are in your moves to use that effectively though.
Hotaru has two different followups, based on whether or not its a counter hit. A counter hit is when you hit them out of a move; usually if you do that they get stunned longer.
Non-counter: all you can do is IAD after them and j.2A > j.C usually, but if you're close enough to the corner you can do Hotaru > land > IAD j.C > land > Gurren > 2C > j.2A > falling j.2C > j.2A > IAD j.2A > j.2A > j.C > 5C > 3C. I'm not quite sure if that's 100% correct so you should look at the combo thread.
And for counter hit: Hotaru > land > IAD j.2C > 2C > j.2A > IAD j.2A > falling j.2A > 2C > j.2A > IAD j.2A > j.2A > j.C > 5C > 3C.
Both of those are execution intensive.
As for 1 star combos or combos that are cheap, those you can get off a 3C counter hit (CH), counters, or any anti air.
You should go look at some combo videos in the video thread, or watch some videos of JP Hakumens playing and take your combos from them.