Rule #1

Excerpt from "Using Staves" [Magic Items -> Staves -> Using Staves][1] says

Staves use the wielder’s ability score and relevant feats to set the DC for saves against their spells. Unlike with other sorts of magic items, the wielder can use his caster level when activating the power of the staff if it’s higher than the caster level of the staff.

Rule #2

Excerpt from "Magic Items" [Magic Items -> Saving Throws Against Magic Item Powers][2] says (Emphasis mine)

Staves are an exception to the rule. Treat the saving throw as if the wielder cast the spell, including caster level and all modifiers to save DCs.

Question

I find these contradictory, so how can i determine the Spell DC and / or Caster Level of a spell cast via a staff? Also, since all staves are at least 8th CL, wouldn't these be too powerful for low-level chasters? 8 rounds of summon monster I (1 round / level), 8 hours of pass without a trace etc.

Staves are the exception

I can see where, in the semantics, this can be a bit confusing. Let me go over caster level first.

The simplest application of caster level (CL) to a magic items comes with wands and scrolls. Fireball, for instance, does 1d6 per caster level, so a scroll of Fireball with a CL of 5 does 5d6 damage. It doesn't matter if it is in the hands of a 20th level Wizard or a 4th level Barbarian who's trying to use magic device. The same is true for any level-dependent variable in the spell's description.

Now, as your quoted rules state: staves are an exception to this rule.

So--
Rule 1

the wielder can use his caster level when activating the power of the staff if it’s higher than the caster level of the staff.

Rule #2 (altered by me)

Treat the saving throw of the staff as if the wielder cast the
spell, including caster level and all modifiers to save DCs.

Spell DC

The Spell DC and / or Caster Level of a spell cast via a staff is the character's own caster level or spell DC if it is higher.

Let's take the Staff of Fire (to continue with my fire example). The CL of the Staff of Fire is 8. Now if I were playing a Wizard of level 8 or lower, this is the CL that I would use and so it would cast a fireball for 8d6. If I had a level 11 wizard wielding it, I would then cast a 10d6 fireball.

Saving Throw

From Staves (emphasis my own):

Staves use the wielder’s ability score and relevant feats to set the DC for saves against their spells. Unlike with other sorts of magic items, the wielder can use his caster level when activating the power of the staff if it’s higher than the caster level of the staff. This means that staves are far more potent in the hands of a powerful spellcaster.

So, regardless of whether it was my level 8 Wizard or level 11 Wizard using the Staff of Fire it would be a save DC of: base 10 + 3 for a 3rd-level spell + casting ability score modifier.

As to your last question: whether the staff would be to powerful for a low-level caster...

You can Use Magic Device

Use a Wand, Staff, or Other Spell Trigger Item: Normally, to use a
wand, you must have the wand’s spell on your class spell list. This
use of the skill allows you to use a wand as if you had a particular
spell on your class spell list. Failing the roll does not expend a
charge.

The check would be 20, which for lower level could be quite difficult to attain, certainly there would be a few wasted rounds.

It's not contradictory, staves use the wielder stats

Those two pieces of text are saying the same thing, in different ways. Staves always use the wielder's stats and caster level to define the effects of spells cast from the staff, differently from every other magic item, which use the item's caster level and minimum ability score to cast the spell to define the DCs.

That means that a summon monster I spell cast from a staff by a 3rd level caster will last 3 rounds, but if cast by a 8th level caster it will last 8 rounds.

Note that for staves, you must have that spell on your class's spell list and be of high enough level to cast that spell so you can recharge it, otherwise you can use those spells freely. So, if a staff has summon monster I and summon monster IV, for example, a 3rd level caster can use the SM I spell from the staff but SM IV will have the minimum duration of 7 rounds (the minimum CL to cast SM IV). On the other hand, a 8th level caster could use both.