Where can I store my altcoins? I was thinking of buying and trading some altcoins, but can I just store them in like for example bittrex? Or should I download the altcoins individual wallet?
It's tough, because there isn't one wallet that accommodates all currencies. I'm sure there will be someday, but not today.
What you can do is evaluate the wallets that do exist and see which may store all the currencies you own or at least narrow down the options.
Here's a
Wallet Comparison site that also displays the currencies supported for each wallet.
Good luck to you!