Arrancarization

[Eloni]

We know that only VLs have human (or human sized) form naturally (http://manga.bleachexile.com/bleach-chapter-197-page-16.html ) and that Aizen can create Arrancars from Menos with the help of the Hogyoku.
http://manga.bleachexile.com/bleach-chapter-188-page-12.html
http://manga.bleachexile.com/bleach-chapter-188-page-13.html

We also know that Aizen has the Hogyoku for a couple of months (since he took it out of Rukia) and he'd made lots of Arrancars since then.

But in the Neliel vs Nnoitra battle Nnoitra asked “How many years do you think it has been since I first kicked you out of this place (LN)?”.
http://manga.bleachexile.com/bleach-chapter-292-page-9.html

Nnoitra, Neliel and Szayel clearly had human form in the flashbacks but they aren't VLs (OK, we don't know this for sure in Nel's case, but Nnoitra and Szayel couldn't be VLs since it's stated that a VL's power is greater than a captain's -http://manga.bleachexile.com/bleach-chapter-197-page-16.html - and they were defeated by Kenpachi and Mayuri pretty easily).

Isshin and Urahara mentioned pre-arrancars in chapter 188 but they also said that they were imperfect. Isshin said that the arrancarized Grand Fisher was at a completely different level compared to the pre-arrancars that they’ve seen before and that GF has grown into “such a powerful monstrosity”. Could Nnoitra, Neliel and Szayel be those pre-arrancars they were talking about?
I highly doubt it since that would mean GF was more powerful than them. It would be ridiculous since even if Nnoitra and Szayel had become more powerful since then, Neliel couldn’t grow in strength because she was rendered to a child but after her transformation she put up a good fight against Nnoitra.

So if Aizen hadn't had the Hogyoku years ago how is that possible for an Adjuchas to have human form? Is there a natural way to gain human form? Or did I miss something?

[Saffire]

Remember Volume 3. Grand Fisher was turned into an Arrancar without the Hougyoku, by other Arrancar who had presumably done the same thing. Also, Isshin says during his speech to Urahara that "the level of the Arrancar, which hadn't changed for decades, suddenly took a major jump". So yes, Arrancar predate Aizen using the Hougyoku.

[Rain]

Well isshin and urahara said that the levl of the arrancar like GF is higher than what they knew of before

I don't think this means anything, as i bet it was just referring to arrancar from regular hollows, not menos and above.

arrancar like the privaron, nel, nnoitra and szayel are all natural forming arrancar


so just because Isshin didn't know of powerful or complete arrancar this doesn't mean they didn't exist

and Urahara said to Ichigo and co that the only menos guarenteed to have a human form post arrancarization is a VL, but other weaker levels can have it

so we only know one thing for certain, if it isn't like a human form, it isn't a vL

so pretty much to answer your question, we know of gillains and adjuchas who had a human form

[SenorGhost]

When Hitsugaya was explaining the class of Menos, he was talking about unhybridized hollows. All Arrancar should ideally take a more or less human form. Take Grimmjow's fracción for example. Aizen stated that they were Gillian class and yet they all appeared to be very human like (Since Grimmjow's flashback chapters show them as Adjuchas I have assumed that they reverted to Gillian class).

The released forms of Nnoitra, Szayel Aporro and Neliel, In my eyes, are not humanoid. This is their actual hollow form, and is something that will not change regardless of the completeness of the Arrancarization. As they appear when they have sealed their power, again, will be rather humanoid.

Just another note about the Grand Fisher comments Urahara made: There was no indication that any of the old Arrancar under Aizen's command had ever been observed. Remember, this is Sōsuke "Manipulate all of Soul Society" Aizen, it's not beyond him to keep these Arrancar from being observed. The imperfect Arrancar mentioned may not have even been hybridized by Aizen. Whether the Arrancar Isshin and Urahara referred to were created with Aizen's help is to be seen, but either way it's clear from Nnoitra's statement that Aizen had created powerful hybridized hollows long before he obtained the Hōgyoku.