Is it ever okay to say Kaddish when there are not 10 Jews together?
No. Kaddish is considered a davar shebikdusha, and may not be recited unless a minyan is present.
The only instance in which kaddish can be recited in the absence of a minyan is the kaddish that follows the Amidah prayer, when there were ten people present for the prayer.
Sources: See Mishnah Berurah, 55:2, 7, 17, 20, 22.