I wish they had those car seat covers when my daughter was a baby..shes 16 now! The recommendations are for not using any sort of additional layers in car seats. Not sure how parents are expected to do that with all small children, but with babies you easily can by tucking a light blanket around them snugly and using a good one of these car seat covers. Really, if they are in an infant seat, you will be going in and out of the car/stores/house. In the "olden days" (LOL) we tossed a good blanket over the whole baby or car seat and it was a big thing to lug around and try not to get dirty in sloppy store conditions, etc (carts and junk..ugh).
Fleece is good in layers as it blocks out wind and is warm and easy to wash...but as a provider, I see many of those covers have a flannel exterior and a sort of fluffy fake sheepy looking thing inside. . The babies are always toasty warm when they arrive at my house (I do home childcare).
Just consider and read up on current recommendations for car seat safety and adding layers with bulky coats or bunting sorts of things for babies. Alot of opinions out there, but really, the seats were not designed for bulky layers to be added and for the straps to still function at maximum safety. They are meant to be snugly next to your child with normal clothing.
As for your home, keep it at what ever your normal temp is. At my house, we have always kept it cool. Always. And now, as a teen, she still likes a cool house, year round as well, but thats just us! LOL. The sleep sacks are great they have now..many price ranges and thickness, etc. Good old sleepers with legs for bigger kids as well. Again, speaking as a provider who has done various safety training...SIDS traing tells us (and is regulated for licensed providers) that for children under 12 mos of age, we are not allowed to have ANY blankets in the cribs of our daycare children..so these sleep sacks are great! Especially in a cool house like mine. Of course for a parent and your own home, you get to decide! I keep my house 66-68 year round (colder at night in the winter for just my family..we go to 58 at night for us)..and my daycare babies always have a onsie, good socks, long sleeve shirt and pants....nothing unusual and they stay warm enough!
For outdoor play, etc for older baies/toddler/children..I love Lands End and can't say enough good things about their stuff. As a child my daughter lived in their stuff...and still has the womens boots (I have jackets of theirs)...and an old jacket and snow pants to use for sledding fun.
Good Luck and Enjoy Winter!!! I do!