Say I pay the life to activate {immolation souleater}s ability does something like {kiln fiend}s effect kick in ?
No cause that isnt a instant or a sorcery. Even though mana abilities have instant speed they arent instants.
Also say I have two {immolating souleater}s on the field and I pay 2 life for one of them to make it a 2/1 does the other get the effect and become a 2/1 or do I have to pay 2 life again ?
You have to pay 2 life for each one plus 2 more for every use. (you can use its abilities more than once a turn. U could make it a 3/1 if u wanted for 4 life or a 15/1 if u pay 28 life)