You have 3 effects, probably 4 here.
First you activate Evo-Force, it will resolve normally since no chain has been formed. You tribute Najasho (as a cost) and special summon an evolsaur monster from your deck.
After Evo-Force resolved, Najasho will try to activate as soon as it can (mandatory effect), in this case, during that evolsaur's succesful summon window. At this time a chain will be made (SEGOC) if you summoned an evolsaur that triggers when summoned by an evoltile. Let's say you summoned Evolsaur Vulcano with Evo-Force:
Vulcano and Najasho will try to activate at the same time, simultaneous effects go on chain: link 1 Najasho, link 2 Vulcano. After Vulcano resolves, Najasho brings out an evolsaur monster from your deck.
To sumarize, no...you cannot activate Najasho before Evo-Force finishes it's resolution and brings out your first evolsaur. What you CAN do is summoning Diplo with Force, and building your chain with both Diplo and Najasho.