Is it possible to set it [1] to display F keys by default (and switch to alternate functionality only on pressing Fn key)?

[1] System Preferences > Keyboard > Keyboard > "Use all F1, F2, etc. keys as standard function keys" option on El Cap.

No. You can set the Touch Bar to show, by default, "App Controls with Control Strip", "Expanded Control Strip", or "App Controls". And you can change Fn so it shows "Expanded Control Strip" instead of the F-keys (e.g. if you never need F-keys). But you can't set the Touch Bar to default to F-keys and require Fn to do something else.