Quote:
and I want to do quarter note triplets for chords in a few parts of the song on the first two beats of certain bars

You can change the Style at any bar. Hopefully that might deliver what you need, but that is limited to an entire bar, not part of a bar.
For example: start with a 4/4 style. At the required bar, press function key <F5> and select a 12/8 style, then repeat the process at the bar that you wish to return to a 4/4 style.

Alternately you could possibly create your own style using MIDI that meets your requirements.

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