I have every quat. Indio mandarinquat is poor, and really nothing like a mandarin. The skin is thin, dry, and not really sweet. Inside is sour and has a slight bitter taste like Rangpur lime. Nippon orangequat is like a bland sweet orange with no noticeable kumquat heritage. It is large, with puffy skin that is not sweet but just bland
If you want something like mandarin-kumquat the closest you will find a regular old calomondin. I eat them whole and when they are very ripe they are pretty decent... but still sour.
If you want a tasty kumquat I highly recommend fukushu/changshou. Marumi, meiwa, and seedless nagami are great too.