Having read through almost this entire thread (pg. 267), I can't find anything to suggest it's use is in any way hazardous. The carts are what come in to contact with your concentrate and are made of stainless steel w/ a ceramic heating element, if I'm not mistaken. They were made with concentrate in mind, so they are probably going to be your best bet in terms of on the go vaporization. So yes, relatively safe.
The Herc is an attachment primarily for Herb, with an added oil option for thinner oils. You do not need it unless you want those two options. You just need a regular cart (when you look at a Persei, it's what looks like a metal straw).
I don't know what to tell you about QWISO. The guide says that you should be able to use bho, earwax, putty, shatter, co2, nitro extracts and the like. I would think it depends on the consistency and the purity of your QWISO. If the consistency is right, i.e. solid at room temperature, melts with heat, you should be able to load it up, but if you have bits of plant matter in the QWISO it'll probably clog and eventually ruin the cart prematurely.