I'll use their behaviour from the anime to try and guess who would they be into. Usually people fall in love with someone else who has a trait they lack in life. So people are in a way like puzzle pieces. They wouldn't like someone who's exactly like them.
Let's jump into it: https://twitter.com/itsalecastro/status/1248032440595161098

Formaggio is a laid back, relaxed person who loves to joke and watch football. He's a simple guy really, he's direct in his fights as well and not manipulative. He is also sensitive to remarks. He's living in the moment. What he needs is someone that is orderly a bit.

Formaggio, on a long term relationship, would love someone who plans for a future, looks ahead and helps him set his life in order. Surely a person like that would nag him but that would show him that they care about him. His life won't be all over the place. It's balanced.

Onto Illusio, he's a perceptive man and opposite to Formaggio he seems to be a planner who plans not only his fights, but also his demeanour. Thing is, he keeps that false demeanour on and if he liked anyone, it would be someone who can see through it and make him love himself.

Illusio acts like he is confident but he seems to be timid and doubtful about himself deep down. His insults at Formaggio may be a reaction to how he feels about himself and if anyone can help him with that, it'd be a kind person who teaches him how to develop while being himself

For Pesci, it's obvious what type of person he admires. He's not confident, talks more than acts, yet is very kind and cheery so what he needs is someone to get him on his feet and cheer him on to for him to actually overcome his fears and accomplish something with one he loves.

Pesci needs someone who is quick and steady on their feet. Pesci would love a person who always encourages him physically and wordly. He would like a team worker too, not an aloof person as he would love to share life goals.

For Prosciutto let's get one thing out of the way; he doesn't care about how a person looks. His uniform and looks are all because he sticks to Squadra's reputation as professionals and cares about their motos by the book. Never once did he comment about his comrades' appearance.

Prosciutto is someone who has seen how old, fragile, and ugly one will get and thus he cares more about other traits. Potential and mind. The two things he was working on to change Pesci. Prosciutto is a serious man that won't stand anyone that doesn't take him seriously.

He has too much control and I think he'd love a person who challenges him mentally and ability wise. Someone who takes him seriously and works with him as a team player but doesn't follow blindly. He's a mentor and is open to be mentored himself.

He'd fall in love with a person who teaches him new things and life perspectives. A clever person that would learn from him and teach him.
If Prosciutto is with a weak willed person like Pesci for example, the relationship would only corrupt him making him overlook his weakness.

Onto Melone...gosh the thing about Melone is that he looks at people as if they're science specimens. He seems to be very quiet, polite, non confrontational, and only gets riled up once the subject of his scientific interest comes up.

Melone also doesn't care about social norms. Due to the way he looks at women with disregard and 0 interest, I fully believe he's gay. The only person he'd find as his equal and possibly have feelings to would be another male that's also as expressive as him.

I'm gonna straight up say that Ghiaccio needs someone who is able to listen to him. The man is smart, prideful, and has many interests that he's vocal about. He doesn't need someone that would fight him, but rather listen. He'd like a chill but smart person that can keep up.

Finally Risotto. To be honest, canon Risotto is too broken to dedicate himself to someone else. He is broken beyond repair but to answer this let's assume its the Risotto from 1999 (who had all of his team/2nd family alive)

Risotto is a very calm man that's calculating and can read people easily, however, it is because of this calm demeanour he always has emotional bursts. He acts like a good father in a way noticing if someone is missing, keeping financial issues and problems to himself..

He also seems to like living under one roof with many people that he cares about which implies he'd be someone who would like having a big family whether he's with a man or a woman. Honestly, I think he would fall for a person who wants a permanent future with him.

Risotto lost many things in his life so a stable relationship is what he'd like where he has a home and a family of his own since he lost his when he was young. A caring, serious about future, and vocal person would do. If he traps everything in, a vocal person can ease him.

So yeah, people love others that can complete them. This mini essay is about how a lover can complete squadra members but remember that they too should complete their lover so both would have strengths and weknesses.
Who would you complete, hm?