Like Phy - my boyfriend moved in with us right at the start of our Vee relationship and it has worked out fine. However, in our case MrS (my husband) and Dude (my boyfriend) were already best friends before the Vee formed and Dude was already here 3-4 days out of the week for 6 months or so.
We all sleep in the same bed (unless one of the boys opts for the couch) but if we had the room I think each person having a sleeping/personal space of their own would be ideal (especially if they are going to be dating outside of the Vee - which hasn't happened for us yet but could in the future.)
I think, though, that you don't really know what it is like to live with a given other person until you actually do it. Should probably have a back-up or transition plan in case it doesn't go smoothly.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe