It is better to say Du`a between Sujud in the words narrated from the Prophet which are ‘Rabb ighfir li, Rabb ighfir li.’ It is permissible to add to the Du`a between Sujud that was narrated from the Prophet or using words other than those that were narrated.

If the shirt includes writing that is not prohibited but it attracts attention and distracts the worshippers who look at it, then it is disliked to pray in it in that case, otherwise it is not disliked. But if the writing carries prohibited meanings, it is not permissible to wear it at all, whether outside the prayer or when praying.

The basic ruling on moving while praying is that it is makruh unless it is done for a reason. There is no specific number of movements that invalidates the prayer, rather it is any movement that contradicts the idea of prayer in the sense that if anyone saw this man doing that it would look as if he were not praying.