Before you mention the Battle of Mu’ta as evidence to support sending a small group of Mujahideen against a vastly larger force, bear in mind that the Muslims were not aware of the full size of the army of the Kuffar until they reached the battlefield, and once they reached it, they debated turning back.
Upon the beautifully inspiring words of Abdullah bin Ruwahah, they charged into battle. The Muslims were in severe danger until Khalid bin al-Waleed took command and maneuvered a tactical retreat. Upon their return to Madinah, the people blamed them for “running away” but Rasulullah ﷺ defended their decision on the basis of the ayah:
“وَمَن يُوَلِّهِمْ يَوْمَئِذٍ دُبُرَهُ إِلاَّ مُتَحَرِّفاً لِّقِتَالٍ أَوْ مُتَحَيِّزاً إِلَى فِئَةٍ فَقَدْ بَآءَ بِغَضَبٍ مِّنَ اللَّهِ وَمَأْوَاهُ جَهَنَّمُ وَبِئْسَ الْمَصِيرُ”
There is nothing in the Seerah, and nothing in the Fiqh of Jihad which sanctions deliberately confronting a vastly larger and better equipped force that cannot realistically be overcome militarily.
Obviously, the Battle of Badr does not contradict this, indeed, it confirms it. Both Abu Bakr and ‘Umar advised against confronting the Quraysh on the basis of the unpreparedness and inadequacy of the Muslim forces. Rasulullah ﷺ, however had been assured of victory by Allah Subhanahu wa Ta’ala prior to the battle, therefore overriding the otherwise sound reasoning of the Shaykhayn. Of course, we do not have such assurances today, and must depend upon objective appraisals of comparative power.