I saw the Messenger of Allah (may Allah's peace and blessings be upon him) praying while sitting cross-legged

"Sitting cross-legged" means to place the sole of the right foot under the left thigh and the sole of the left foot under the right thigh, and sit on the buttocks. The Prophet (may Allah's peace and blessings be upon him) did this when he fell off his horse and his ankle was dislocated.
So if the praying person is unable to stand in the obligatory prayer, it is recommended that he prays while sitting cross-legged, and this is in case he is sitting instead of standing. As for the sitting between the two prostrations and his sitting during the Tashahhud, it is recommended that he sits in the posture of Iftirāsh (placing the left foot on its side and sitting on it; and keeping the right foot vertical while resting on the bottom of the toes turning them towards the Qiblah) in the first Tashahhud and the posture of Tawarruk (sitting with the left posterior on the ground; the right foot placed vertically with the toes pointing towards the Qiblah; and the left foot on its side emerging from under the right foot) in the second Tashahhud.
All of these postures are recommended. So, if one sits in a posture other than the ones mentioned above, it will be sufficient for him since what is required is to sit during the Tashahhud, and these postures are a level higher than the obligation.