It’s very difficult to prove a negative, but I am reasonably confident that no rule in 3.5 or Pathfinder explicitly states that all armors of a given weight class slow you down by the same amount. In 3.5, at least, there were certainly a few super-heavy armors that reduced the speed by more than your typical heavy armor (Races of Stone had a couple of these).
So yes, every Medium or Heavy armor must somehow explicitly state its own speed reduction, even if the overwhelming majority of them have the same speed reduction. On the other hand, if a given case failed to do so, I’d just chalk that up to the designers not realizing there was no such general rule, since it usually is so consistent and armors which slow people more or less than others in their category are quite rare. It’s also worth keeping in mind that it is consistently weight category that we should consider, not sheer weight.
Mithral Chain shirt would be Medium (treated as light for all but proficiency)
The relevant parts of Mithral are:
Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.
The relevant part of Kilt are:
When you add an armored kilt to a suit of light armor, the set counts as medium armor. Likewise, a kilt and medium armor counts as heavy armor. Adding an armored kilt to heavy armor has no effect.
Firstly lets address: "I would like to add that the armored kilt table itself does not mention or specify any movement reduction." Once you stop using a kilt as a piece standalone armor the rules stop caring about what category of armor used to be. (making Mithral kilts irrelevant) It is now part of the new armor (Sean K Reynolds says that you cannot stack enchantments with a +X on a kilt. He is the best authority we have for now, and it supports common sense)
A kilt adds one to the armors category for all purposes when you attach it (including movement). If you somehow managed to make a kilt that was medium and it had the above rules text it would still only add one to your armors category. Noticeably, this means you require proficiency in the new category of armor. (Medium for the Chain shirt)
Mithral does not actually modify the armor category. It allows you to treat it as lighter for purposes of movement and other limitations. Mithral and Kilt have no actual interaction.
In summary:
- Kilt: Add one to armor category for all purposes
- Mithral: Pretend its one lower for all restrictions (except proficiency)
Thus the following combinations are possible:
Gear: Treated as for restrictions(Proficiency required)
- Light Armor w/ Kilt: Medium(Medium)
- Light Mithral Armor: Light(Light)
Light Mithral Armor w/ Kilt: Light(Medium)
Medium Armor w/ Kilt: Heavy(Heavy)
- Medium Mithral Armor: Light(Medium)
Medium Mithral Armor w/ Kilt: Medium(Heavy)
Heavy Armor w/ Kilt: Not possible
- Heavy Mithral Armor: Medium(Heavy)
- Heavy Mithral Armor w/ Kilt: Not possible
To throw in a little common sense
The kilt should probably be made from Mithral if the armor is made from Mithral to allow Mithral to apply. This is in no way RAW however.
Additional Source
Best Answer
His move speed remains 30 ft. in both 3.5 and Pathfinder.
3.5 and Pathfinder use identical wording for what mithral armor does:
While there's some room for disagreement about what "other limitations" means, movement is explicitly called out. Thus, a mithral breastplate is unambiguously light armor for movement purposes, and light armor doesn't slow you down in either 3.5 or Pathfinder.