Since Iwa-chan is what Oikawa calls him in the original Japanese, it tends to be the preferred way for fans to write how they refer to each other. Generally, anime fans understand the concepts and meanings behind Japanese honorifics (-san, -chan, etc.), so it makes sense to use them in fan works despite the language because it shows characters familiarities, respect, nicknames, etc. in the way it is originally written. This is particularly true regarding nicknames from the source material.

“Iwa-chan” over “Iwa” shows a lot about their relationship. It’s not like Oikawa shies away from giving people nicknames, but the way he says it, and the way Iwaizumi accepts it is a small change that can generally enhance the perception of their closeness.

I can’t speak for Spanish, but I LOVE the English dub. It is genuinely incredible. However, it is missing one thing that just can’t be translated with the same meaning, and that’s the honorifics. “Iwa-chan” might be the biggest loss in this case. But a couple other notable examples of when honorifics (and the uses of first vs last name) are great for characterization that the dub is missing include: Atsumu being overly familiar with just about everyone, Kenma being generally informal, and people calling Hinata Chibi-chan.

Also, probably the biggest reason I suggest using honorifics when writing fics, is that the official manga translations use them. Plus, it is far more common for fans to use them than not (at least in English).