You don't have to worry about the amount of slots...
As far as I know , there are slots for 1000 kits. I think, it's enough for all the missing kits, at least home, away and GK:
Premier League: 20 teams, 2 licensed teams -> 18 team kits to create: 54 kits
Championship: 24 unlicensed teams -> 24 teams kits to create: 72 kits
Ligue 1 and Ligue 2: completely licensed
Serie A: 20 teams, only two unlicensed teams -> 2 team kits to create: 6 kits
Serie B: 22 unlicensed teams -> 22 team kits to create: 66 kits
Eredivisie: completely licensed
Primera Division: 20 teams, 2 licensed teams -> 18 team kits to create: 54 kits
Segunda Division: 22 unlicensed teams -> 22 team kits to create: 66 kits
Liga NOS: 18 teams, 2 licensed teams -> 16 team kits to create: 48 kits
Liga Brasileirao: completely licensed
Liga Argentina: completely licensed
Liga Chile: completely licensed
League PEU (example: for Bundesliga): 20 teams -> 60 team kits to create
League PLA (example: MLS): 20 teams -> 60 team kits to create
League PAS (example: J-League,...): 20 teams -> 60 team kits to create
(several teams of different leagues already in-game -> reduces number of kits to create)
National teams: 83 teams, +/- 10 licensed teams -> 73-75 team kits to create: 225 kits
Summary:
225 + 60 + 60 + 60 + 48 + 66 + 54 + 66 + 6 + 72 + 54 = 771 kits
ML fake teams: ca. 30 unlicensed teams -> ca. 90 kits
=> 771 + 90 = 861 kits to equip all unlicensed in-game teams
Then we still have around 140 kits for third kits or second GK kits.
So I think, it's better to create kits with a higher resolution of 2048x2048.