Is it forbidden to imitate clothing designs from companies, factories, or the Internet and sell them, even if they observe Islamic guidelines and one can add simple, innovative details?

Inventions and innovations are protected by Islamic law, and it is impermissible to infringe upon them.

If what you are doing, by taking clothing designs, is considered an infringement on the rights of their owners, such as taking their registered trademark or trade name, then it is not permissible.

However, if they are merely designs that are similar in appearance but differ in trade name and the like, then there is no blame upon you.