But, most of these villains had appeared before in Strange Tales, which featured the Human Torch in solo tales (and later, partnered with the Thing)!
the Masters of Evil, which must have been enough to get Pete out of prison. Pete didn't learn his lesson, returning with new weapons to face the Human Torch and the Thing in Strange Tales #124 (September, 1964 in "Paste-Pot Pete" by Stan Lee and Dick Ayers), where Pete causes trouble for the Thing and the Human Torch....
Sandman then teams up with five other Spider-Man foes like Kraven, the Vulture and Electro to form the Sinister Six in Amazing Spider-Man Annual #1 (October, 1964 in "The Sinister Six" by Stan Lee and Steve Ditko, proving that the Sandman was willing to be a team player...).
Medusa was there when Maximus made Black Bolt unleash the power of his voice (which was quite destructive) in Thor #149 (February, 1968 in the flashback tale "Silence Or Death" by Stan Lee and Jack Kirby), when the Inhumans moved their city in What If? #30 (December, 1981 in "Moving Day" by Peter Gillis and Ron Wilson), and then in the Inhumans Special #1 (April, 1990 in "Remembrances of Revolutions Past" by Lou Mougin and Richard Howell) wherein Maximus caused Medusa's amnesia, and she ended up in Europe as a thief, and met the Frightful Four.
...at which point, the Frightful Four realized that they'd need a new fourth member...
Spider-Man and the Human Torch in Marvel Team-Up #2 (May, 1972), they then reunited and tried to add super-strong Thundra in Fantastic Four #129 (December, 1972)...
...then began a long search for a fourth member, which included characters like the Brute, Electro and the Klaw among others, and the tales of the Frightful Four get stranger!