Fuuma vs Emiya(Assassin)

In a nutshell, Kotarou is the better star generator and has an AoE NP and Kiritsugu is the better boss killer.

Kotarou, however, remains a 3-star. And really, where Kiritsugu generates about 35-40 stars on a brave chain, Kotarou generates 40-45 on a quick brave. It's not a huge difference. Both of them have a skill which can keep either themselves or the DPS alive a little longer.

And honestly, Hans is a decent star generator unto himself. Not as good as an assassin, obviously, but he fills the role nicely. He would be the one to replace if all you want is more stars.

Fuuma has slightly better star gen due to his ability to quick chain as his deck is q q q a b with a quick np.
Kerry has q q a a b deck with an arts np.

If you have an arts based team, obviously Kerry is the better choice. His targetable taunt + instinct skill (generate stars on use) is great too.
Fuuma isn't as good in an arts/buster team but has good utility due to his targetable Dodge.

Would personally pick Kerry as I have better art teams and he generates more burst star due to his taunt + instinct skill.

Looks like you got a lot of answers about Fuuma vs Emiya, so I'll just leave that alone and try to answer your other question (about how to work a star generator into your team). Personally, I would *not* replace Hans like someone else suggested to you. Hans gives your team a +40% crit damage steroid, so if you're trying to go with a crit team, you obviously want to keep him in there for stronger crits.

Mash not only does not give your team a crit steroid, but she also has an unusually high star absorption for a support, so she will steal like half of the stars that you generate. That being the case, you'd want to replace her with your star generator, if possible. Mash is great if you need additional survivability in a fight, but Hans' +1000 health Regen per turn and his defense buff usually can give you plenty of survivability for standard fights (especially since you'll be killing stuff faster by critting their faces off).

The think is, Fuuma on paper has better Stargen, but Kerry has a better deck. In Arts team, Kerry wins obv, and in Buster team, you should use support that has passive star skill and equipped with 2030 and the likes. Not saying Fuuma is bad, it's just 4 Quick cards is not a good thing for Buster/Arts team card pool.
For Mash Hans comp, if you have passive stargen CE (2030 and the likes), equip it to them. It's just your DPS hopefully is an Archer/Rider, because Mash has standard Saber/Assassin star weight.