I don't understand why you think it would be helpful for either of Darling's partners to testify. They are not the parent, they are not the step-parent, and I don't see how bringing your boyfriend's other partners into a custody hearing could do anything but hurt your appearance to a conservative judge.
If the courts won't even let your ex visit the kids without supervision, then they sure won't give him custody any time soon. So he would have to claim that the kids are in such a bad situation that they would be better off placed in foster care. That usually only happens if you're doing something harmful or neglectful.
I would also recommend contacting a social worker and explaining your situation. My friend had an abusive ex who tried to have her kids taken away. He got as far as getting Child & Family Services to come do a home inspection. They basically concluded that she, as a single mom on social assistance, needed more help. Rather than taking her kids away, they provided her with support and programs. Talk about backfiring, as far as the ex was concerned. If a social worker does an evaluation on your family and finds that the kids are not at risk of harm, then they can testify on your behalf. That will carry much more weight than your boyfriend's wife.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker