adoptee shiva

Question

I understand that there is no chiyuv of shiva and no mitzva of shiva for an adoptee. But is there any ISSUR? Surely it is noy ossur to sit on a low stool and refrain from showering. Is it ossur to tear ones clothing? There may be a problem with refraining from pirya v'rivya, if that applies, but surely there must be things that the adoptee is allowed to do even if he is not required to .