They added ability to use eCredits with companion certs awhile ago. Yes, RUCs can also be used on Companion certs. They can also be used for other members of your party even if booked on a separate companion cert from you as long as they are on same flights. Recently used one companion cert on spouse and one child and used RUCs to upgrade them and have myself and other child on another companion cert and will try luck with comp upgrades on flights (flying out on Christmas Day).