im planning to make my lee's 100% fire dmg
is that good idea?

just remember that dmg is dmg, regardless what type it is. it is the attack strength that makes the special effects happen (i.e. ignited). So you will do more dmg, but you will not ignite them any more often with only dmg mods.

If you had a 20% poison damage mod and a 15% fire damage mod you should put in the poison one

Also, the more different types of damage you have the more possibilities you have for stun, poison, phase, and shock

Only if you equip + attrack strength mods.
A poison damage mod won't ever poison an enemy unless you're got +poison attack strength from somewhere.