So many GDAs!
materialtypes is where tint, material ruleset, and default sound-type are set for each material (there are a crapload more columns in this GDA, but the rest are defunct).   By default, clothing materials do not have rulesets, so I have made some new ones, by extending materialrules.gda. 

materialrules sets the base offensive and defensive properties for any material.  For the new robe materials, I simply copied select lines from leather because it was easier than attempting to balance things myself.   For the stat nerds:
Tier 1 (apprentice/tranquil) robes are based on cured leather.
Tier 2 (mage) robes are based on hardened leather.
Tier 3 (senior mage) robes are based on inscribed leather.
Tier 4 (first enchanter) robes are based on dragonwing. 
ALL mage robes use these tiers, Circle or apostate. 
 
itemstats determines which material properties are used for each item type (for instance, staves and armor can both be iron, but staves read the columns for "damage", "damage range", "armor penetration", "aim delay", "optimum range", & "spellpower"; while armor only looks at the columns for "armor", "ability cost" (fatigue), and "range penalty"), and the multiplier (medium and heavy armors both use iron, but heavy has a higher "armor" multiplier). 
Clothing by default has a 0 in every stat; I set it to have an armor multiplier of 1 instead.  To compare, light armors (leather) have a armor multiplier of 3.
Even though they may have no associated material ruleset, all clothing items now have a minimum of 1.0 armor rating.  Cloth helmets have an minimum armor rating of 0.25. 
 
bitm_ contains even more item properties, mostly related to inventory and usage.  Runes are set here, in the "Rune Count" column. 
Rune Count here + the RuneCount column in materialrules = actual number of rune slots on the item. 
Item types that are not supposed to have runes (like staves or shields) are given a Rune Count of -100 in bitm_ to make sure they never get runes based on material.  I set clothing to have a baseline of 0 runes, like other armors. 
There are mods that give rune slots to clothing (like this one); these work by setting the Rune Count in bitm_ to 3.  If you want to have both slightly armored clothing AND guaranteed three rune slots, use the bitm_ from that mod instead of mine. 
(It actually might give up to 6 rune slots on high-level robes; IDK, I haven't tested it) 
In addition to having RuneCount set in bitm_ and materialsrules, each individual item must also be rune-enabled, via a little true/false flag in the UTI. Unfortunately, this flag is set to false by default, and must be flipped, either by UTI edit or by script.  Dain's Fixes includes a script that enables runes whenever you acquire an item.   If you don't have Dain's, I've also included a runscript to fix all items in your inventory (so long as that item should have runes based on material and type): "runscript enable_runes".