blocking the plate or a base
 

this has nothing to do with a collision at any base.....with that said.....when can a catcher plant his foot infront of the plate (third base side)......i mean actually plant his foot.....before he catches the ball or after he has control of ball .......so...again....when can the catcher actually and physically plant his foot in front of the plate (third base side)

Depends on the rules in use.

OBR and NCAA: As soon as the ball is close enough that he can be considered to be in the act of fielding.

FED and LL: Not until he physically has possession of the ball.

F2 (or any fielder) can plant his foot anywhere he wants. The question is, does doing this impede the runner. If it does, then you refer to Rich Ive's answer.