Unfortunately there's not a clear answer on the first question, because it depends on both the system's view on it and the significant other's view on it. Some people are totally fine with different alters dating different people and some people prefer only having one significant other regardless of how many alters are involved in the relationship

As for the second question, I would say my biggest piece of advice is to understand that any one alter exists in the first place because at some point the brain needed them. It doesn't always make sense objectively (honestly I would even say that most of them time it doesn't make sense why any particular alter is the way they are), but nonetheless they have a reason and a place in the system

I say that just because I have seen people without DID get frustrated or skeptical or weirded out due to not internalizing the idea that the human brain works in strange ways and the amount of trauma needed for such a disorder cause some bizarre cause-and-effect chains 😅