Okay here goes:
1. For me there is a lot of dead space that doesn't really add anything to the image. Appreciate that this was obviously taken at distance and it looks like you've deliberately opted to frame to get the trees and a bit of sky in but for me everything above the players isn't adding anything to the shot.
2. Much tighter crop which I think is better. Maybe a little too tight at the top. The major downside here for me is the lack of the ball in the shot. Generally I'd recommend having the ball in pretty much every 'action' shot. If you are doing a stock photo of a player just standing there then you won't need the ball. Similar if it's a celebration shot or there is some form of emotion or incident being captured. For me if you get the ball in it just adds so much more interest and context to the shot.
3. This is good action. You've got a little unfortunate that the players aren't facing you here as that would really have elevated the image but obviously that happens when photographing football. Again the downside for me is the cropping. A lot of space above and below the actual action that I don't think gives the image any value. You want to keep the eyes on the action and getting rid of a bit of sky and grass would help to do that.
4. For me you have the focus on the wrong player here. I think this works better if the focus is on the player facing the camera. In terms of cropping here it is tighter which is great but I always prefer to see more (or equal) space above the players than below. Otherwise it just looks a bit off.
5. Good action shot. I really like this. Again I'd probably highlight the amount of empty space above the players. Not a lot going on up there and I don't want my eyes wandering up into that space so it's better simply not to have it. Take half that space above the players away and it's a much improved photo.
6. Yes, I love this. This is generally the type of shot I like to see where you have two players facing the camera as they contest the ball. You could have cropped a bit tighter but this for me is a pretty nice football shot.
7. This doesn't really do much for me. Neither player is facing the camera which is pretty much a no go area unless you are capturing a key moment such as a goal or bad foul and you desperately need a picture to put out. If you take that picture from the other side then it's going to be a solid action image but from behind I'd have just left it out.
8. Not a bad picture but it looks like you haven't quite managed to frame it to get both the heads fully in the shot? I know it sounds picky but the little details are the things that improve or detract from an image. Those kind of things you start to notice when you compare your pictures to others.
Solid effort for a first attempt though! Bare in mind that what I've written above is my opinion based on my experiences. Almost every photographer can find ways to constantly improve and evolve. When I'm being harsh on myself I can usually pick apart every single image from a gallery and find ways I could have improved it.
I’ve no idea what generally the pros shoot aperture wise and it’s just a personnel thing from me,
Usually f/2.8 using a 400mm f/2.8 simply because we do want the background to be blown out of focus. Similar on the 70-200 f/2.8 but I'll often stop down to f/4 for closer up action and set plays etc to get a bit more DoF.