Is it permissible for a Muslim to pray with their arms at their sides when tired, as the Shiites do?

Placing the right hand over the left in prayer is a Sunnah (definition). If one feels tired during prayer, they may let their hands hang loose to rest, then return to clasping them. The Prophet, peace and blessings be upon him, used to pray at night, and if he became tired, he would sit, then stand again for bowing. If it is permissible to abandon standing, which is a pillar, due to illness, then it is all the more permissible to abandon a Sunnah due to fatigue. This is not considered imitating the Rafidah as long as it is temporary and for a valid excuse. It would only be considered imitating them if it were adopted as a permanent practice.