I believe that they're two different things.

Junior stables are stables under 6 months. From the LB description for junior stables "Determined By: Average horse stats of living foals (That the stable bred regardless of who owns them), has logged in during the last 5 days, has been a Game Member for less than 181 days, and having least 4 living foals."

Small stables have less than 200 horses and stay on the small leaderboard. Description "Top Eventer Breeders - Small Stable - (<200 Bred Horses)"

"Determined By: Average horse stats of living foals (That the stable bred regardless of who owns them), having less than 200 living foals and has logged in during the last 5 days."

Medium stables "Top Eventer Breeders - Medium Stable - (200-1000 Bred Horses)"

Large stables "Top Eventer Breeders - Large Stable - (1001+ Bred Horses)"

So I think to answer the question, junior stables and small stables are two different things, and you can go over 200 horses as junior stables relates to age of the account and small stables relates to the size of the account.

Happy to be told I'm wrong but that's my understanding :)

You will be a junior stable until December no matter how many horses you have. Junior Stable leaderboards and Adams Junior Auction are open to you. Once you hit your 6 month anniversary, you can never be a junior stable again, and you lose access to the Adams Junior Auction.

The 200 horses thing is for small stables, and anyone is eligible for that.