I read at Ampage's tube data for example, on 6L6's.

For Class A1 triode operation
grid2 connected to plate

Is that all about?
That easy?

A DPDT (two power tubes in my amp) swith is keeping me appart
from having pentode - triode options?

yes.

just make sure you retain the resistors that are connected to the
screens (g2). in other words, don't ever run your screens without
some stopper resistance.

try to get the best dpdt switch you can find. even though you're not
switching a lot of current, you'll have nearly the highest voltage in
the amp on it.