Dysfunctional Rules VIII: When General Trumps Specific

[ShurikVch]

Try a Stoneblessed Hairy Spider: who doesn't like nonmagically growing and shrinking five size categories?

You wouldn't like that spider when it's angry!
Fine-sized creature turning into Large-sized while raging is just good
Huge (or even Colossal ) creature - into Large - is just weird: what's up, it get smaller while raging?
And even worse: it says "height only increases by a foot or so";
if creature was smaller than Large, then it's OK;
if creature was already Large, we can say it's "foot or so" taller, but still Large
if creature is Huge or larger, then there is no excuse

Is something still equipment if you can't equip it, though?

They are in Chapter 7: Equipment, so, unless there are some RAW which says they aren't, they are equipment!
I will say more: since services are in the same chapter, they are equipment too, and could be suddenly "found" by Shade Hunter of sufficient level

Also, I'm not sure that Stoneblessed gives Powerful Build in the first place?

No, it doesn't - class description don't have any mentions of Powerful Build

Alternatively, take the entropic and half-troll templates.

Doesn't Entropic Creature template change type to Outsider for any living creature?