Am i able to edit the rest of the default teams and keep my 8?
No, but you can save as many teams as you like and reload them when desired.
if i was to give the editing a shot myself am i able to edit all players on every team including the team?
You can make 20 player per team and, as mentioned, save as many teams as you want.
Do i have to mod my 360???
No.
Note: if your goal is to recreate teams and have them play each other (cpu vs cpu) you can create 20 players, put 10 on each team (plus 1 legend on each team) and this will work. More players than 11 may not work (freeze game).