Resistance and "aiki" in the same breath is an oxymoron...
By definition, "aiki", meaning "to unite/join with another's 'ki' or force", implies being in accord with your opponent. And since you are in unison with your opponent (there is no opponent!), there can be no resistance.
"Resistance" is the result of your opponent sensing your force and opposing it. It is the opposition of forces between you and another that creates resistance. Like an electrical current passing thru a wire, the impedence in the wire creates resistance, and resistance creates heat.
Or magnets... like poles repel, opposite poles attract. Yin and Yang.
So, if there is resistance, it means at least one of you isn't doing "aiki".
Therefore, there is no need for resistance training in aiki, the absence of resistance is aiki itself.
Which vaguely echos the words of Chogyam Trungpa... there is no need to struggle for freedom; the absence of struggle is in itself freedom.