Hi K.. ;) I put Xander in thermal pjs (supertight fit keeps them insulated) and then fleece pjs overtop (not the sack, but pjs which would keep the warmth closer). Plus socks and the wool diaper cover. We keep our house at 68, but you could also add an undershirt/onesie underneath all that and an additional armless fleece sleep sack on top. I am guessing they would let us know if they were cold, but I worry too. Oh I also threw a heavy quilt on top of the crib sheet, tucked into the sides- so he sleeps on top of it, not under it or anything...but it's warmer than a sheet you know?
Also, we have all the heat going into Xander's duct from the basement with his vent wide open and the rest of the house's shut. That helps because that way we aren't heating the whole house, just his room needs to get to 68 for it to turn off.