I was thinking about using the training-boosting items (e.g. Red Mango) on the early stages of a monster's life and see if it's efficient, much like how the training methods in MR2 use Paradoxine/Troron in the monster's baby stages.

Then I suddenly thought, 'what if the effects could last for a month'? Now THAT's efficient.

Of course simple test would be to compare the performance of my monster in Kawrea Volcano in the fall, one run feeding it Red Mango and another without a Red Mango. The monster in question is a 2nd gen Tiger-ish by the way. It has been fed 3 Soda Lozenges beforehand, too.

And basically, it works:

w/ Red Mango:
-Lif + 10-12 for EVERY Parameter Rose panel
-Ski + 4 for every Parameter Rose panel

After a reset and trying again without Red Mango:

w/o Red Mango:
-Lif + 7-9 for every Parameter Rose panel
-Ski + 4 for every Parameter Rose panel

Though the higher gains mean that you'll also receive higher downs (???) from stepping on a Parameter Fell panel.

Is it then OK to use training-boosting items at the baby stage of a monster's life, when it's probably not going to be efficiently do normal training yet and grind them in Drills instead?

It's not worth losing 2 weeks of lifespan to use any training booster items.