it's always good to know a alittle about alot ..then it is to know alot about a little.
But it's better to be amazing at a little than just okay at a lot of things. Be a jack of all trades, yes, but be your absolute best at as many of those as you can...the number will likely be far less than the number of things you can do.
As far as balloons, you don't have to be really good at them. It's not like magic where the entire routine is an experience. With balloons, the amazement comes with the final product. Learn a few of them, like ten or less, and you'll be able to turn a larger profit on your shows. Being a working pro means being flexible to your audience. Word of mouth is huge in this business, especially when you're just starting out, and if you say no to balloons, and another magician says yes, you may have lost quite a bit of business. Giving balloons out at the end of my show has gotten me a lot of business as of late, so I'm still unsure about continuing with them, but now I'm leaning toward yes.