When I got this quest I was under the assumption that I could get ALL the factions to help build it. So I went and talked to the Preston first, and now it seems the other factions won't give me a topic about it. Am I stuck getting the Minutemen to build it? Does this mean i have chosen this faction to side with? I really wanted to side with the BOS. Bummer......

Yes. Once you trigger one faction for Molecular Level, that's the one you choose to build the device. OTOH, it does not limit your final faction decision in game, though others may express disappointment with you for not trusting them.

If you want to change your decision, you need to reload an older save file.

Ok, thanks for your help!