I dunno if those mixed sibkos have ever been mentioned outside of the novels to be honest.
FM:Crusader Clan states "Some Clans breed enough offspring of a prestigious Bloodname or a particular Bloodheritage to fill an entire sibko, though mixed sibkos that contain the offspring of several different Bloodnames are more common. Even though some Bloodnames produce excellent warriors of all types, each warrior type is trained separately to keep competition fierce, yet balanced."
I'm not sure if Ranna was talking about a mixed sibko in terms of just Bloodhouses, or if she was talking about a mixed sibko in terms of Warrior types, but it seems that the regular practice is to keep sibkos pure in terms of Phenotype.
So if you have a particularly bad Kerensky Mechwarrior Bloodheritage, and a bad Ward Mechwarrior Bloodheritage, and you might not want to create 100 of each, you could create 50 of each, and shove them into mixed sibko.
But you wouldn't have a mixed sibko of Elementals and Aerospace pilots for instance. The aerospace portion would probably be decimated after a few years.