New player with questions - formation, squad and player positions

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.

How will you have 25 players in your squad then ?

1. there is no super tactic that can be used to defeat all of your opponents, so along the way you will find at least 2 tactics that can suit you best almost in every match but there will always be bad game where you will think about having 3rd tactic..

2. You can have maximum 22 players total in your team..

3. You must buy a versatile player or train them by yourself into a new role before you put them where they do not belong to.. Because if you do this often then it will make your player to play in a poor perform..