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Magic items like the Staff of the Magi let you cast certain spells from it as an action, including Conjure Elemental which normally has a minute long casting time.
The Staff of the Magi is not the only exception; a more extreme example is the Staff of Woodlands which can cast Awaken this way, which is normally an eight hour long spell.
Is there any resolution to how these different rules interact?
If a specific rule contradicts a general rule, the specific rule wins.
Here, you are giving examples of items that are giving specific rule that contradicts the general rule on casting times.
The DMG even states that some magic items can modify the casting times:
[A spell cast from a magic item] uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration. Certain items make exceptions to these rules, changing the casting time, duration, or other parts of a spell. (DMG p. 141)
The Staff of the Woodlands is indeed one of those exceptional items. It says:
You can use an action to expend 1 or more of the staff's charges to cast one of the following spells from it [...]
The rule for this staff says that you use an action to cast a spell. The fact that one of those spells is awaken (normal casting time 8 hours) does not matter. That casting time is now superseded by the effect of the magic item.
Thus, any similar magic item that has some effect in it that modifies the casting time, will similarly override the general rule. In this way, magic items can indeed allow you to cast spells using a shorter casting time than normal.
Spells (eg Wish), metamagic, and other features already exist to modify the casting times of spells. So magic item use is far from the only example of a specific effect overriding spells' casting times.
Jeremy Crawford also explicitly agrees with this:
The staff of the woodlands allows you to cast the awaken spell as an action, superseding the spell's normal casting time of 8 hours. Some magic items make exceptions like that, as noted on page 141 of the Dungeon Master's Guide (see the "Spells" section on that page).
And he also agrees in the very similar case of the necklace of prayer beads:
A necklace of prayer beads lets you cast a spell it contains as a bonus action, regardless of the spell's normal casting time. For more about how the necklace works, see page 182 of the Dungeon Master's Guide.
The rules for how to use each staff and their capabilities are contained within the item descriptions. For the Staff of the Magi, we see:
Spells. While holding the staff, you can use an action to expend some of its charges to cast one of the following spells from it, using your spell save DC and spellcasting ability: ...
Emphasis mine.
For each item, you will need to read on a case by case basis to determine what the casting time is for the spells it allows you to cast.
This is a great example of a specific rule which over-rides a general rule. This is covered on p. 7 of PHB:
Remember this: If a specific rule contradicts a general rule, the specific rule wins.
In this case, the magical properties of the staves are spelled out in their item descriptions in the DMG as you point out, and must be combined with the descriptions for spells covered in the PHB. These are specific rules that serve to modify the existing descriptions of the spells contained in the staves. In this case, the descriptions of the spells and of general magical items are the general rules that are subject to modification.
The general rule for magic items that cast spells is on p. 141 of the DMG (emphasis mine).
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range and duration, and the user must concentrate is the spell requires concentration.
Since NONE of the staff descriptions in the DMG (pp. 201-207) modify the casting of spells contained within them, the default casting times called out in the PHB must be used.
Two final comments to this:
This is clearly a design philosophy choice. Whenever the authors of the stock magical items in the DMG wished to alter the performance of a spell, they made it an inherent function of the staff, and not an attached spell (e.g. the Staff of Thunder and Lightning's Lightning Strike power, DMG p.203 is not the same as the Lightning Bolt spell on PHB P. 255).
It certainly is within the cognizance of the DM to create a magical item that alters the casting time of a long-casting time spell. This would (in some contexts) be a very powerful item and should be balanced accordingly.
I would also note on Staff of the Magi that all the spells that use charges use an equivalent amount to their spell level, except Conjure Elemental, which uses 2 more charges than you would expect. Based on that, it seems like the designers realized that it would cast quicker. There's some precedent for 2 similar spells where the low level one takes a minute but the high level provides a similar effect in an action. Tiny Hut versus Magnificent Mansion.
