My oldest is almost 21. She's always had more than just mom and dad as parents. My sister was her "second mom" and lived with us almost her entire childhood. She was actually closer to my sister than anyone else (this remains so now).
It was somewhat of a pain in that I wanted my sister recognized for her contribution, but that meant LOTS of paperwork with doctors etc. My daughter ALWAYS wanted her AUntie when she was hurt or sick. So I took the extra time to ensure that every local hospital/dr office etc had on file permission for my sister to do anything for my daughter and she had paperwork in hand as well.
Additionally, my boyfriend has lived with us for the last 10 years. So, all of the kids have been growing up in that dynamic with 3-4 live in "parental role models" and that honestly seems to work VERY VERY well for them. Even friends families (who are unaware of the personal dynamic within the home) comment about "how nice it must be to never have to worry about having an adult on hand".
As for exes-my exboyfriend helped raise my oldest for 4 years (age 2-6) and they remained in contact after I married Maca. In fact, he ended up being a family friend after a year or so. It was deeply meaningful to the kids to keep the adults they love in their lives-even if said adults are no longer in romantic relationships with the bio parents.
My exgirlfriend has also maintained a relationship with my daughter (we dated when my daughter was 2-3). These extra relationships have worked well as back up for me in my duties as a mom. Even though our romantic ties dissolved, I knew these people were people I could trust my child with. So it gave her other safe adults to talk with besides MOM who sometimes (especially as a teen) was NOT her top choice.
"Love As Thou Wilt"