I saw the connector of battery on iPhone se and 6 and 6s are the same!

Well, the iPhone SE has the smallest footprint of all those devices so you won’t be able to physically fit anyone of those batteries inside the SE housing. But yes, you can connect them when the phone is opened up and techs do this for testing all the time.

It will connect, but physically it would not fit in the phone. And i am unsure if the phone would power up with it connected.

Will a battery out of a iPhone 7 fes or a iPhone set a iPhone

I have a spare iPhone SE battery, is it possible to use it on a 6s?