(a) Various materials may be combined to create a Shi'ur Tum'ah since they
are sometimes sewn together into one -- when a person makes a quilted
donkey-saddle (RASHI in our Sugya and in Me'ilah 18a).
The logic of this would seem to be as follows. It is obvious that there is
a sufficient *amount* to be Mekabel Tum'ah, since there is even enough in
the combination to satisfy the largest Shi'ur involved. However, it is not
so clear that different materials can be *combined* altogether; perhaps we
should assume that the materials will shortly be removed from one another,
and therefore we should already view them as *separate* entities.
Therefore, in order to combine materials, we must first prove that the
combination is indeed tolerated. Since people sew together materials in a
donkey-saddle, and since fodder of different types is mixed together when
being displayed for sale, therefore we may view the mixture as a lasting
combination and add together the various amounts to create a Shi'ur.
(b) The rule is that when objects become Tamei, or spread Tum'ah, when they
are similar in size (for instance, if they both spread Tum'ah when they are
the size of k'Zayis), they may be combined to cause Tum'ah (Me'ilah 17a).
Therefore, since, in one instance, the size for causing Tum'ah of *all* the
materials and types of fodder mentioned in the Mishnah is the same, they
may be combined to cause Tum'ah (if the combination is at least equal to
the largest of the Shi'urim involved).
It what manner is their size for causing Tum'ah the same? When they were
specifically touched up to be used for a donkey-saddle, in which case they
are all Metamei when they are 1 x 1 Tefach is size (RASHI Sukah 17b DH
Ho'il, TOSFOS in our Sugya DH Ho'il). Similarly, the amount of fodder for
which one is Chayav for Hatza'ah is the same small amount for all types of
fodder, if it was specifically set aside to be used as a salesman's sample
(Mishnah, 90b).