oh dear, sounds like advice from the fish shop has got you in a right old mess hasn't it.
the vast majority of puffers are aggressive and not suited to community tanks, I don't know exactly what species you have but you can expect trouble to be totally honest.
the behaviour you described at the start is a typical symptom of disease, could be one of a few things. Almost certainly cvaused by poor water quality.
However don't panic, you're in the right place now and we can help you with all this, for starters you need to know a bit about your water. a wise man once said 'we are not fish keepers, we are water keepers, look after you're water and the fish will look after themselves'. it's very true, with good water quality and a healthy balanced diet you can keep most fish happy.
here's some basics on water chemistry
http
/www.fishforums.net/content/New-to-t...eady-have-fish/
next thing to understand is a bit of tank maintenance, here's a step by step guide
http
/www.fishforums.net/content/New-to-t...ly-Maintenance/
have a read of those, get yourself a test kit and test the water, let us know what you're stats are. get a positive ID on that puffer and we can take it from there.