I was a member of fancy rats a while ago (hi acapae! ) nice to see you on here. The build up of amonia (from urine and faeces) in a rats cage can also trigger respiratory issues, so daily spot cleans can help. I get my 20kg sack of aubiose from gjwtitmus online. You can order it from other sources or farm/stable supply shops where it`s probably cheaper. This company deliver next day though so are very quick and reliable.
Glad to hear the single female has some company now that will help her get over the loss of her friend. Rats are great little creatures. It`s just such a shame that they are so prone to respiratory issues during their lives. I would say if you hear any frequent sneezing coming from the rats, it would be a good idea to speak to your vet and maybe get them all onto baytril for a few weeks, or until the sneezing stops, just as a precaution. Sometimes it`s better to catch it early on and not let it manifest into something more serious like a lower respiratory problem. It`s quite normal to hear rats sneezing to be honest just because they are very prone to this, but giving them a good diet and as much variety as possible will keep them healthy and their immune systems strong.
I found my rats like to snooze during the day but will wake up periodically during the day for edibles and drinks. They are more active in the evenings normally.