First off, Welcome to the forum. I would hope that if your fiancee and his new gf have had poly relationships before they'd be a little keener about helping you answer your questions. If not, I think you found a good place for them here...I'm sure some others will jump in anytime.
You do have some important questions that you're perfectly justified in asking...although the actual answers may be a little elusive....keep asking them anyways.
It does sound like they're in the middle of NRE...so he maybe right...it'll settle out eventually....but that doesn't mean you can't make your needs known...nor that you should just sit down, be quiet and 'enjoy it' in the meantime.
If you have questions, ask them. Needs...communicate them. Fears...investigate them. I don't think I've heard of a poly relationship yet that suffered from 'too much' communication.
The upshot...you may have a lovely and fullfilling relationsip coming your way...I'd be remiss if I didn't suggest to embrace it.