Erika, there is a cream you can get from the vet called TriTop that is would be helpful for Gandolph's poor hiney. It comes in a tube and a little dab will do ya...er, him. When I had my hysterectomy Suki had to be kenneled a few days. She picked up some funk there and poo'd till her bottom looks like it was going to bleed.
The cream started getting it healed up in a couple of days.
It could indeed be the giardia again. In some cases, it can be very tenacious. Hugs to Gandy and the rest of the crew, and your family....Hope you can have a reasonably happy holiday...So sorry your furkids are ailing.