New player with questions - formation, squad and player positions

[toren95]

I am just done with my first week in top eleven and think i am beginning to understand most of the game. I have some questions and thoughts i will need advice and personal preferences with.

1. Formation
In all my matches so far besides 1 i have used different counter formations against my opponents formation, this have given me a win in all of them. In the match I didn't use counter, I faced a player who did the same as me in his matches, so went with a flat 3-5-2 as I thought it was a flexible and somewhat safe formation and managed to turn 2-0 to 2-3 with some changes,
Is this the right way to approach all matches and does it work in higher leagues?

2. Deciding the squad
With two 4-4-2 formations i have 22 players, I want to add 1 in each part because of injuries so the squad will consist of 25 players. Is this enough for the league and 1 of the cups thinking of injuries and energy to the players?

3. Player roles/position
How much penalty is it for a player who plays in a position with yellow warning sign?
Because i want that all players should know 2 different positions, so that i can keep a small squad with many opportunities in formations and in match tactical changes without using substitutes.
Almost all of my currently players only know 1 position, training all the youngest players in new positions will take time so is it worth it?

So far i am thinking that all players on the field should know the postion forward them in the 4-4-2 formation, so the DL can play DML, the LM can play AML, MC can play AMC and so on.
The 3 extra players should know 3 positions.

I hope the wall with text didn't scare you, thanks for your time.