I aspire to be a writer of fantasy. Of course, I am going to be writing short stories and other small pieces in order to sharpen my writing skills to the maximum. Now the question is if I should confine my writing to fantasy short-stories. I know that I should be building skills in that genre due to my aspiration in it, but should should I always or 95% be writing in that genre?
Honestly, that's a question best saved for the mirror - AKA yourself. If you're worried about building skills, don't be, as all writing builds your skill set, even just journal writing. If you want to make a name for yourself in fantasy, you can indeed write mostly fantasy, but that doesn't mean you have to be exclusive to the genre. It's more like an open relationship than a marriage when it comes to writing, so don't worry if you aren't true to that one genre. It won't mind!
There are a lot of writers who are single genres creators. Doesn't make them better or worse than those that write outside of genres comfort zones. I love shorts collections, even though I am still working on how to write shorter things better in general. And the long ones too. You just need to decide whether or not you want to write outside of Fantasy. Some find it hard to write outside of what they are comfortable/enjoy writing the most. You can always hybridize other genres into Fantasy. Write what you want, in what ever genre makes you most happy.
Do what you want. Personally, write what you like and don't confine yourself. First of, there are tons and tons of crossover Fantasy/Science Fiction authors. Second, bringing in tools from outside your genre can make work INSIDE your genre better. Historical Fantasies work because they draw from outside the fantasy toolbox and use tools from Historical Fiction. Personally, one of my favorite subgenres is Sci-Fi/Fantasy detective novels - they've become a habit ("Yiddish Policemen's Union," "Osama," "Our Lady of the Ice," "Who Censored Roger Rabitt.") And the great thing about those books is that you really can't get them write unless you have an author who is well versed in both Sci-Fi AND Mystery.
I don't sorry any problem with sticking to that genre if that's what you want. Don't feel confined either, though. All writing is good practice.
I don't read fantasy of any kind. It doesn't appeal to me. My first book was a modern urban fantasy. I know, right? Everything I've written since has been factual, modern-day stuff. I'm still proud of that first book though. I will write whatever I feel compelled to. I write stories, not genres.