Headings should emulate one another, so try to use the term History over anything else. When in doubt, try looking at the format and descrpitive terms used on more populated pages to refresh.
As for meta-explanations on creatures, it's not needed. When you write out an article for a creature resembling or sharing its title with that of another, just tag the other creature with a "See Also" or "For" templates.
"See Also" is best suited for use with similar yet different creatures within the same universe (i.e. A-Virus Zombies, t-Virus Zombies, C-Virus Zombies), wereas "For" is useful for different continuity's having similarly titled creatures/characters (i.e. on the Las Plagas (Anderson) page, "For the game counterpart, see Plaga")
As for only being knowledgable with the films, not a problem! Whenever I or another person here gets around to finishing the books we could always add more info then. As for your edits, only place what you factually know in them... Lots of people stretch what they think happened and tend to fall for blame later when a page is saturated in non-confirmed theories. It's better to only put what you know.
I've updated Las Plagas (Anderson) for you to take a look at, study it and try imputting a similar format into Majini Undead. Let me know when it's finished so I can review it! :D
(P.S. on Majini Undead, try removing overly-descriptive movie lingo. It's not important to the creature's history that there was a unique camera angle on the prison.)