I would wonder if you are getting ahead of yourself here with the baby naming thing. You say that you have been involved with this couple for a year. It sounds like it is long distant. I would think that you don't know what will come of all this enough to start predicting and wondering what it will be like having babies with him. You don't know what it will be like to live with them yet... isn't that the first step?
I would suggest btw that you find your own place before moving in with them. I have heard more often than not on here the break ups that occur when a LDR partner is moved in and the dynamic changes so much that the whole thing is blown out of the water. It is simply not the same thing to visit.
Being independent is important. Having your own money, your own belongings, your own friends, job, car, place to live means that you can have autonomy while you all adjust to the dynamic changing... not to mention that they just had a baby. That is a huge adjustment for them also... you moving in could be just too much.
I see a red flag in your comments about thinking that you would have your own husband, and child and mono life. Are you sure that this situation is right for you? You say he is your love like he will be and could only be the only one. I hate to break it to you, but it is likely he isn't going to be the only one and that you may find that you love another someday. Especially if this situation isn't a good fit for you because you have other plans for your life that eventually need addressing.
It sounds like you are putting a lot of eggs in one basket here. That is a ton of trust and vulnerability and not too much taking care of yourself, following your dreams and goals and adjusting yourself to fit them. You are your own person. Your goals are valid. Your independence is vital to the success of feeling self worth and gaining valuable self esteem. How will you do this if you are in a position where you are last in line to a baby and a wife? He of course would give you some attention, but a mother with a new child is very demanding, not to mention a mother and a child in general... this baby is new... extra demand right now. Oh ya, and there is another child too. that makes fourth in line.
Lastly, am I correct in understanding that they are moving you in and haven't told anyone who you are and what you will be doing there? So not only are you going to be last on his list of priorities but he is going to hide in public with you so people don't know his feelings towards you? When you do come out, all of you, there will be the burden of the family and friends on you I would think. They are not necessarily going to open their arms and embrace you as another wife. They likely will be very judgemental, angry and ostracise you. If you don't have your own place, where will you go to recoup from that kind of treatment?
It seems to me that worrying about what name your future baby four years from now might have is the last thing to worry about. It might be that I have over emphasized some points. This could be a worst case scenario that I have given you... I don't know. I am only going by what I have read here on these forums for two years and the knowledge I have from living 41 years of life... 15 of which as poly...
I suggest you do some reading here. If you do a tag search and find the thread on "living together" you may find some interesting thoughts to consider... here some threads to start