In my opinion i think:
1) I would recommend definately do the leg work in the gym. You cant flex bone! No matter how explosive you are if you have little strength you wont go that far that fast and reach your highest potential. In my opinion the higher your relative lower body strength the faster you will run.
I highly recommend sticking to LOW
reps and heavy
weight, especially for the main exercises like squats, deadlifts, bench, military press etc. So for football players i would recommend almost always sticking between 3 reps to 8 reps at the maximum (normally) ( i would mostly stick around 5 repsish)
This is because anything over maybe 12 ish reps wont make you much stronger or bigger. Just make you sore and increase your stamina a bit. BUT tbh i think if you want to improve stamina you would be much better off sprinting, doing intervals, playing footy.
Anything around the 8 rep mark will make you stronger, but you will mainly just get bigger. This is know as sarcoplasmic hypertrophy. This isnt ideal for a footy player as they wont to get as strong as possible while gaining as little weight as possible. ( This does not mean muscle mass is bad, as a bigger muscle is normally a stronger muscle BUT you want to focus on increasing strength.
The 5 rep range will give you an icredible mix of muscle and strength.
3 rep range will be mainly neural gains. ( strength gains)
Relative body strength is key for footy.
Rep speed wants to be controlled on the way down on the negative. Then as fast
as possible on the way up! We are athletes we want to be quick and lift quick. ( obviously when you are using heavy weights you wont be going that fast, but try and go as fast as possible.)
Going slow will slow down gains and increase recovery time. ( especially on negative)
In terms of sets more sets are needed for muscle building and less for strength training normally.
Another thing to note is dont try and force out extra reps ( e.g. partner help) and dont go to faliure too much.
This is so you can stay fresh for your plyos, footy practice, sprinting, endurance training etc. ( and next strength session)
Finally in terms of injury prevention
Warm up before exercise
Make sure you have a balanced body ( pushing and pulling equally matched, dont just focus on e.g. chest or quads ignoring back and hammys)
Improve flexibilty and soft tissue quality
other things just cant think of atm.
Any questions just ask!