Say, when using the Create Staff feat, are we able to just throw spells into it willy-nilly (with DM's permission, naturally...), or do we have to use pre-defined sets of spells? For instance, could I just make a staff that has all of my stat-buffing transmutation spells, even if no such staff exists in the source books?
I don't think it's on the SRD, but at some point i did pick up some expanded item creation rules, not sure if they're official or not, but they seem to work well enough.
Staves are crafted the same as Wands, you can put as many spells of as many different kinds as you like into a Staff, but there are restrictions on charges as follows:
By default, spells use 1/2 their level in charges (rounded down) with a minimum of 1 to cast. So level 0 -2 spells cost 1 charge, level 3 - 5 spells cost 2 charges, level 6 - 9 spells cost 3 charges. If a spell has an XP cost, it costs an extra charge for every 5,000 XP, minimum 1 charge.
When creating a staff, you must choose the spell school it belongs to. Spells that do not belong to the same spell school as the staff can be put into it, but cost 1 extra charge to cast. Spells that are anathema (prohibited school) to the staff's school can be placed into the staff, but cost twice as much to cast as normal. (So putting Ressurection into a Necromancy staff would cost 10 charges over the base 5 (3 for level 7 spell, 2 for 10,000 XP cost)
To get the price of a staff, work it out the same as a wand for each individual spell and then add the results together. Note that a Staff has a minimum caster level of 8.