I read they can be used in some of the older Kayfun tanks but the fact they use pg or vg means they’ll still be overly sweet.
ALL ejuice is made up of vg, pg, nicotine, and flavorings (and/or sweeteners which is in the flavoring category). These are the main components in e juice no matter where you get it from.
Nicotine in its pure form is deadly. So, it is suspended in (mixed with) either pg or vg prior to sale, so that its safer to handle.
Then, the company who makes your e juice cuts the nicotine further by mixing it with more vg or pg (or both) and add flavorings as well, creating the e juice you purchase which then has a 'safe for vaping' level of nicotine.
how sweet an juice is has less to do with your vg, and more to do with what an e juice company (or DIY enthusiast)
adds to it by way of flavorings and sweeteners. While VG has
some sweetness to it, the added sweeteners are more the culprit of overly sweet commercial e juice than vg ever will be.
As a consumer of commercially made e juice, you simply need to look for companies that don't add an excess of flavorings and sweeteners to your e juice, if you want to avoid overly sweet.
For an example: my husbands tobacco flavored e juice isn't sweet compared to my cinnamon roll e juice. The difference isn't in the amount of vg, as we both use the same amount, but rather, the difference in sweetness has to do with the flavorings and sweeteners I ADD to the base vg/nicotine solution. That said, would his tobacco flavored e juice likely be even less sweet if I were to use more pg in his e juice? Yes, it is possible, but the difference is also likely to be unable to be noticed in such small quantities because flavorings and sweeteners have more influence in how e juice tastes. I could make you some 100% vg e juice that you would swear had not an ounce of sweetness in it, for example, simply due to what flavorings i could add.