My last name and my younger son's last name is Sch**f*r. My older son's last name is L*f*bvr*e. My fiance and his children's last names are K*lt*n. My older son is 17, so he's ready to make a life of his own- he has his own last name and doesn't care either way what everyone else is called. The baby's father has a different last name than the baby, and he has minimal involvement in the baby's life, but it would be a cold day in Hell before he consents to letting me change the baby's last name to anything but HIS last name, which is not happening. So... when I marry my fiance, my last name will be S******r-K****n and everyone else will keep their last names. Any future children will have the last name of K****n, like my fiance. If, in the event I can eventually change my youngest's last name to K*****, I will drop the Sch**f*r part of my last name and just go with K*lt*n.
In your case I would probably keep my name, have fiance keep his name, and then hyphenate any future kids' names. I could also see giving future kids just his or just my name though, it could work either way.
In my family, I am the only one with a different last name-it hasn't been a big deal, but I don't know how your fiance would feel about it. My dsd has the same last name as my kids and her dad, but has a different last name than her mom/stepdad + siblings at her mom's house and she seems okay with it-she will occasionally sign her name as Dad's last name-Mom's (new) last name on homework, etc. When she was little, she sometimes insisted that her last name was the same as her mom's (mom reverted back to her maiden name after the divorce, then took her 2nd husband's name when they got married-confusing, sorry!), but it was not a huge issue, at least at our house.
All that to say, don't overthink it :) You would probably get used to the sound of the hyphenated names, or you could unofficially use any combinatino of names without making it legal, etc. Good luck!
After my XH and I split up, I changed our DD's name to be hyphenated with both of ours. Now I am due with Baby #2 with my husband...the baby will take his last name and I have my last name and his last name hyphenated...so at least I have both the names of both my children. It gets complicated!
My 4 kids with my ex husband have his last name, as do I. When this baby comes, she will have my fiance's name. When we get married I'm going to legally hyphenate my last name, so that I'm easily identifiable as all of my children's mother, but in day to day life, I will likely just use my fiance's last name. It was the simplest solution, because it made me sad to have a different last name than any of my children-especially since they're still young. My ex would never consider letting me change the kids' names to my maiden name or my new married name once remarried-especially since our son is named after him.
If your girls are OK with changing their name when you get married and change yours, it's definitely worth at least asking your ex. Worst case scenario, he says no.
You may not have to track him down at this point- it would probably be considered abandonment by the court, therefore you would win by default.