Angelo doesn't mean physically, although that's certainly true - their bodies fit together perfectly when they move together, and when they lie curled together afterwards.
But that's not what he means.
Paige is the chirpy one. Never annoyingly, not when you love her, but Angelo understands how it could be.
Jono goes to equal - if opposite - extremes. Nobody would accuse him of chirpiness, and his gloom can get wearing.
Angelo likes to think he balances his lovers, emotionally speaking. But he knows they're all necessary to their dynamic.
They fit. It's as simple as that.