I hv a 4 AAA battery pack and a 2 18650 battery pack but i don't have a 12v battery...so i wanted to know if it is alright if i added both the packs and supplied 13.4v (also will it destroy the board?)
You should never mix different types of cell in the same battery. All cells should be the same type, same manufacturer, same history.
When the softest cell goes low the other cells will push it negative. If that happends to a litium cell it is toast.