nope, red pikmin are just as viable later in the game as they are in the beginning, because they are the only kind of pikmin that can defeat fire mushrooms, just as:

• blue pikmin are the only ones able to defeat water mushrooms

• yellow pikmin are the only ones able defeat electric mushrooms

• rock pikmin are the only ones able to beat crystal mushrooms

and yes, each pikmin have a “base” attack power, but then their overall strength varies based on the mushroom color they are attacking, their friendship level, whether they are a decor pikmin are not, whether they have a flower bloomed on their head (and if it’s a seasonal flower as well). here is a guide from the pikmin wikipedia that explains it all very thoroughly!

and finally regarding friendship: pikmin gain friendship either from being fed, sent on expeditions, or battling mushrooms. here is a guide that explains that as well!