I am not sure if you're asking if all those films should be in one giant collection or if the collections should remain as they are since your penultimate sentence essentially made your case, "all the other titles are [...] spinoffs and their own part".

A spin-off is set in the same universe, and may have some of the same characters, though it doesn't necessarily follow the same plot or timelines; it begins a new series, its "own part". This was discussed a lot last year in more detail, unfortunately a lot of the pertinent threads are gone from the data loss. Basically, there is only one criteria for a collection: the film has to be a sequel.

Anyhow, was Predator 2 a sequel to Predator? Yes. Is Predators a sequel to its Predator predecessors? It seems so,

Rodriguez and Antal have expressed that they wanted this film to be a sequel only to the original Predator[6]12,[13] as the film is trying to distance itself from the two Alien vs. Predator films.[5][12] Antal stated the reasoning behind the decision to dismiss the Alien vs Predator films was that he wanted the film to be closer in tone to the original Predator film, as the AVP films had taken the Predator series in a too outlandish direction.[13] -Wikipedia

Excuse me please, english isn't my native language. It's possible, that i wrote something misunderstanding.

IMO i would put the whole "Alien" Universe just in a list and drop the collections but off the Alien Quadrilogy. In this case it's absolutely sure, that this is an series, an chronological order. But i don't see this really on the predator spin offs, or prometheus or the AvP movies. Do you understand, what i mean? I can not see any series character to any of these shows, so that i am questioning public, if those collections make sense, or not.

Under no circumstances i want to see all those movies in one collection! In this case i could just put all the Gamera, Godzilla, Rodan, Mothra and King Kong movies just to a huge "monster" collection. ^{^}

I am thinking at this one:

"The intention of collections is quite simply to group a set of sequels. 1.Sequels are generally the only valid criteria for collections. Please use lists to group films based on any other criteria. 2.Collections are not mirrors of DVD or Blu-ray box sets, use lists to imitate box sets."

http://www.themoviedb.org/talk/512d31b4760ee35a5d0016df