I have swaddled 2 week old DD for the first time tonight as we always have problems getting her off to sleep at this first stage of bedtime (no real problems after later feeds, just this time of night).
So tonight I've swaddled her in a cellular blanket. Her room is 20 degrees at the moment and she is wearing a short sleeved bodysuit and a babygrow.
Does that sound about right? She'll go into her sleeping bag after the next feed at 10/11pm or whenever she wakes.
We use a Miracle Blanket (we did use a sheet but DS was getting too big and strong for it) and have DS in just a short sleeved bodysuit underneath -- but then our flat is warmer than yours (we had problems getting it cool enough even during the cold snap) so yours sounds about right. If it's hotter or when he had a slight fever we took a layer off him and had him just in his nappy.
If she seems too hot when you unswaddle her then try using a sheet or else dropping the babygrow next time, but you'll probably find it's just right.