Wa ʿalaikum al-salām wa raḥmatullāhi wa barakātuh
Straightening the rows is an important part of the congregational prayer; the prophet (ṣalla-l-Lāhu ʿalaihi wa sallam) instructed us to straighten our rows and warned us of the consequences of not doing so.
This important sunnah is achieved by aligning the heels (not the toes) for those that pray standing, and for those that pray sitting by aligning their backside with the heels of the other congregants.
As for those that pray on a chair: If they perform the qiyām and rukūʿ standing and then sit for the sujūd and tashahhud, they should align their heels with the rest of the row, which means that the chair will have to be placed behind the row. This is the recommended and optimal way of doing it, however, if placing the chair behind the row does not leave the person in the following row enough space to do rukūʿ and sujūd in a comfortable manner, then the chair needs to be moved further up until there is enough space for him to comfortably perform rukūʿ and sujūd.
Regarding the one who prays sitting on the chair the entire ṣalāh, the chair should be placed on the row by aligning the hind legs of the chair with the heels of the other congregants.
As for the placing of the hands during sujūd and tashahhud, they should be placed on the thighs close to the knee by aligning the fingertips with the edge of the knees.
Allah knows best.
 Based on what imam al-Ramli (May Allah shower him with mercy) says in Nihayat al-Muḥtaj that those who are not able to perform proper sujūd because they prayed laying on their sides or on their backs should keep their hands placed on their thighs during sujūd just as they do in tashahhud.
Answered by Shaykh Omar Mohsin