Legal Question in Intellectual Property in Texas

The copyrights in the created work, ie, the anime characters, arose in the creator at the time of creation. The copyrights include not only the right to prevent others from copying the characters but also to prevent others from creating derivative works. The slightly modified characters that you mention would be derivative works. A derivative work is generally any work you create that is based upon the original character. The only way to get around this is to create a new work that is not based upon these characters. The alternative is to obtain a license or purchase the rights to create the derivative work or to copy the original works from whomever owns these rights. Unless these characters entered the public domain, which would be unusual, the answers are (a) no, (b) create a completely new character, (c) very likely, you will need permission from the owner of the copyrights. Note that the copyright owner's ability to enforce the copyrights is a different issue but assuming the characters are popular, which is why you would want to place them on shirts, the copyright owner can likely enforce the copyrights and likely has an interest in doing so.