Can I repot my Echinocactus grusonii, Mammillaria elongata, Mammillaria gracilis var. fragilis, and dragon fruit into a mixture of Miracle Gro and Perlite?
You could, but you'd be better of using some kind of grit if the plants are to be outside, seeing as its more natural that way. I normally start with a based of loamy, black dirt, then add, in equal measure, course sand and grit ( I've ended up using crushed quartz, but I've used heaps of other things, once even crushed up bricks). Go steady on the fertilizer.
Cocopeat or coir are also good to add to the mix. Most ready made potting soils use peat and it's not as easy to re-wet after it's dry, as it probably will be with the cacti. I add ground coir when I mix up my own cactus soil.