As this happened to a guppy it could be Dropsy
and like bloat it is a bacterial infection, dropsy is not usually contagious, unlike bloat, but the cause can sometimes be due to water qualityand as these were new fish the problem could have been caused where you got them from. High nitrite levels can be a cause so if you can test to make sure yours is OK then you can rule it out.
If your other guppies show no signs of it in a couple of weeks then you should be OK to add the Pepps.
How often are you feeding the "Super" protein pellets, the high protein contents will cause problems for BN's so should only be fed a once a week.