1) "MIPNEI SHE'HEN RE'UYIN LI'TAMEI B'MOSHAV"
OPINIONS: The Gemara quotes the Mishnah in Me'ilah (18a) which teaches that various types of material can combine to make a Shi'ur for Tum'ah (l'Chumra; the total size must be equal to or greater than the Shi'ur of the material that requires the largest Shi'ur of the blend), because "they are [all] fit to become Tamei with Tum'as Moshav." Similarly, the various types of fodder mentioned in the Mishnah here combine to create a Shi'ur for Hotza'ah, because they are "fit for a sample." What is the logic of these reasons for why different types of things combine to make a Shi'ur?
(a) RASHI here and in Me'ilah explains that different types of material may combine to create a Shi'ur for Tum'ah, because they are occasionally sewn together into one usable item, such as a quilted donkey-saddle.
Rashi's logic seems to be as follows. It is obvious that there is a sufficient amount to be Mekabel Tum'ah, since there is enough material in the combination to satisfy the largest Shi'ur involved. However, it is not clear that different materials may be combined to make a Shi'ur in the first place; perhaps the materials stand to be separated from each other shortly, and therefore they should already be viewed as separate from each other (and thus do not combine to make a Shi'ur for Tum'ah).
Therefore, in order to combine different materials, we must first prove that the combination is indeed tolerated. Since people sew different materials together in a donkey-saddle, and, likewise, since different types of fodder are mixed together when put on display for sale, the mixture is viewed as a lasting combination and the various amounts combine to create a Shi'ur.
(b) The rule is that when different types of objects become Tamei or spread Tum'ah, they may combine with each other to make a Shi'ur for Tum'ah when the minimum Shi'ur of each one is the same (for example, they both spread Tum'ah when they are the size of a k'Zayis; see Me'ilah 17a). There is one case (as will be described below) where the size necessary to cause Tum'ah is the same for all of the types of material and types of fodder mentioned in the Mishnah, they combine to cause Tum'ah (as long as the combination is equal to or greater than the largest of the Shi'urim involved).
In what case is the size for Tum'ah of all of the different materials the same? The size is the same in a case in which the materials were specifically touched up to be used for a donkey-saddle. In such a case, every type of material is Metamei when it is one by one Tefach in size (RASHI to Sukah 17b, DH Ho'il; TOSFOS here, DH Ho'il). Similarly, the amount of fodder for which one is Chayav for Hotza'ah is the same for all types of fodder when it was specifically set aside to be used as a salesman's sample (Mishnah, 90b).