I love using the sleeping bags. they don't recommend putting extra blankets, but alter what baby is sleeping in. I have no idea how, at age 19 months, what he would do if just under a blanket - he is such a wriggler at night that he manouevers round from one end of the cot to the ohter.
At the moment DS is in a summer weight bag, but i have the winter weight too.
Take last night, it was chilly, so I put him in long sleeve pyjama top so his arms didn't get cold, tonight it is humid and muggy - so back to sleeping in short sleeved pj top and bottoms.
HTH
Alex