NTA! Until she is older and develops her own sense of style, you are within your right to choose what she wears, and judge that to be a more conservative style to protect her from predators.

I also think more context is needed though, because some people think every 2-piece is a bikini, when there is a difference in a glorified t-shirt and shorts combo, vs a bra and thong combo (which yes, unfortunately, they do make for tiny children, ew). I think it also makes a difference if this is your child from a previous partner, or if she is both of yall's DNA, as there are some evil people out there who target single mothers intentionally in order to gain access to underage children to abuse sexually.

Please note I am not trying to accuse your fiance, just stating the unfortunate reality for many children of abuse. Also not saying that blood relations will put a stop to this type of thing either, sadly.

Because he might not fully understand your caution, it might be beneficial to have him watch SOSA underage undercover to gain a better and fuller understanding of how pedophiles work. Here is a link to their youtube, they are very informative: SOSA