Though they may use "magic portals" and "pocket worlds", the Infershia really isn't extra-dimensional, merely underground. (besides, even Magitopia uses pocket worlds)
Likewise, I sort of wonder if some of these villain groups aren't "multiple" in nature. (particularly the Evolian and Jashinka, both of which came to Earth as microscopic organisms and technically are as part of their homeworld as they are invaders due to their nature of evolution being influenced by their environment)
I think that in the Magiranger universe, it's split three ways: Earth (the basic surface world), Magitopia (the "heaven" of this world) and Infershia (the "hell", basically the underground). The gates that go to Infershia essentially go underground, and even with all the special ways to get around it it is still basically the underground. I don't think there is any extra-dimensional qualities in accessing either realms, it's just realy difficult.