Every language DOES have a developmental order, and those orders are unique, meaning that Italian’s developmental order won’t be the same as English’s. There *may* be similarities between similar languages, so I wouldn’t be surprised if Spanish’s developmental order closely resembles Italian’s, but I don’t know if there’s enough research on those languages to confirm that.
Once people learn about this stuff, there’s a tendency to look for “optimal” content. If I’m not ready for irregular past tense verbs, then perhaps I shouldn’t read anything with an irregular past tense right now. If I can’t handle the subjunctive, then I shouldn’t encounter it yet.
But that’s not actually the take-away from this developmental-order research stuff.
The main point I wanted to make with this research is that adults and kids learn in roughly the same way. The similarities between the learning order in adults and kids demonstrate this. If kids learn from exposing themselves to lots of comprehensible input, then adults can do it in the same way. And kids AREN’T selecting input that’s exactly within their current developmental cycle. They’re getting exposed to ALL SORTS of sentences with ALL SORTS of complexities, and they grow from every single sentence they (mostly) understand.
This means…that’s exactly what you should do. Learn sentences from ALL parts of the language. If you’re not in a position to master the past tense, learn it anyways. It will help you grow, and what’s more, it will teach you a new grammar rule. Even if you’re not ready to MASTER that rule right now, you can use that grammar rule to understand more from new sentences, which means more input coming into your head, which means faster progress.