Any effects that modify a creature's power and/or toughness without setting them to a specific value (i.e. ones that don't affect base power and/or toughness) will apply after its base power and toughness are set, regardless of the order those effects were created. The same is true for counters that modify its power and toughness.

Tanazir Quandrix's last ability overwrites all previous effects that set a creature's power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after the ability resolves will overwrite this effect.

As the last ability resolves, the base power and toughness of other creatures you control are set to Tanazir Quandrix's actual power and toughness, not just its base power and toughness. If Tanazir Quandrix's power or toughness changes later in the turn, the other creatures you control aren't affected.

To double the number of +1/+1 counters on a permanent, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.