Does a small amount of blood from a permissible animal (halal), such as sheep, that gets on one's clothes, nullify ablution (wudu')? And is it permissible to pray in those clothes?

Ablution is not invalidated by blood touching one's clothes or body, regardless of the amount. It is permissible to pray in clothes stained with pure blood, though it is preferable for a Muslim to pray in their best attire and appearance. As for impure blood, it is not permissible to pray in clothes stained with it, unless the amount is small, in which case it is excused.