We've used sleeping bags since DD was 10 weeks old. We have used a mix of Grobags, Dream Bags and Living Textile sleeping bags (depending on which ones I could get on sale, lol). DD has 2 of each TOG rating - 0.5, 1.0 and 2.5 in her size. She's currently in the 18-36 months bags which seemed so long when she first went in them (as with all her bags) but they need the growing room. You just leave the excess bag laying flat and lay your baby in the middle of the cot.

I personally only use the bags without sleeves and those with a TOG rating. I feel a lot safer knowing exactly what temp DD room should be and what she should be wearing. The temp and clothing guide which comes with TOG rated sleeping bags are excellent and take away most of the guess work.

Currently we live in an old house with very little insulation so DD's room gets really cold. We have a reverse cycle aircon in her room so that runs all night to keep her room at around 18 degrees in winter. She wears a 2.5 TOG bag with either a Wondersuit and a spencer (so her arms have a double layer of clothing) or she wears a PJ set with a long sleeved onesie underneath. Both options work well.

I wouldn't feel comfortable using the bag you're describing overnight but only cause I'm a worry wart A lot of the soft, fleecy type bags can actually be quite sweaty and don't breathe well so despite having the baby's room at a good temp, they can overheat. I'd only use that type of bag if we were out and about, in the pram - where I could keep an eye on her temp. But that's just me

With regards to blankets and bags, most sleeping bags state that you should not use sheets or blankets in addition to a bag. They shouldn't be necessary if your child is dressed per the recommendations and their room is at a good temperature range. However, if you don't have heating in their room, I can see how it would be needed but I personally would use a heater before I'd consider extra bedding.

HTH!