Logically his dimensional device should not allow him inside DC Universe or what was the point of refusing Cisco help. And MC is going to be OP so he should not form Very large team. Just choose few Characters he can help reforming or few who do not mess up Universe Big Events due to their absence. He can take few characters who would be very powerful if developed properly with appropriate support. Like Jewellery Bonny from One Piece with aging power or Karin Uzumaki from Naruto for sensor, healing and binding etc. Little character with small screen time.