A few spiked chain questions.

[Hypersmurf]

However, in terms of disarming, I am not sure if you would get a -4 to disarming for it being a small weapon or not.

You'd get the -2 on all attack rolls that you mentioned, and you would lose the +4 bonus for wielding a two-handed weapon. So compared to the Medium Spiked Chain, the Small Spiked Chain would be at an effective -6 for disarms.

By the rules, a colossal giant could grab a pixie longspear and he'd threaten, what, 50 ft?

As written in the core rules, Small and Medium creatures wielding a reach weapon double their reach. Large creatures wielding a reach weapon of appropriate size double their reach.

What this means for Colossal creatures is undefined. But the pixie longspear (or, legally, Huge longspear) is not a reach weapon of appropriate size for the Colossal giant. So does his reach increase? It's not clear.

That's why I use a modified Savage Species system. Reach weapons of a given size category have a 'reach modifier'. Small and Medium weapons are +5', Large are +10', Huge are +15', etc. Tiny reach weapons have no reach modifier, but for Tiny creatures, they allow the wielder to attack adjacent opponents.

So a human with a Large longspear threatens 15', but not 5' or 10'. His 'one square threat zone' has been pushed outward 10 feet. An ogre with a Medium longspear threatens 10' and 15', but not 5'. His 'two square threat zone' has been pushed outward 5 feet.

A Colossal giant who somehow wielded a Small longspear would threaten 10' through 30', but not 5' - his 'five square threat zone' would be pushed out 5 feet.

-Hyp.