"しか" and "だけ" are similar way of thinking with "few/little" and "a few/ a little" in English. i.e. "しか" is negative perspective and used in negative sentences, as you wrote examples above. On the other hand, "だけ" is positive perspective and used in affirmative sentences. There is only one fact, but there are 2 ways to describe the situation, either pessimistically or optimistically. 私は、やさいだけ　たべられます　＝I can eat only vegetables. 私は、やさいしか　たべられません＝I cannot eat anything but vegetables. 今日は、　ピザだけ　あります ＝ There is only pizza today. 今日は、　ピザしか　ではありません ＝ There is not anything but pizza today.

Kara limited the situation in a question you gave on her answer. Therefore, I would like to add one more comment. Imagine that you are invited to a dinner with Japanese foods. And you are looking at Udon, ra-men, Soba(Buckwheat noodles), Sushi, Miso soup, and Tsukemono(pickle) and so forth. But you can not eat only Japanese pickle in these menus. In this case,you can say that ' つけもの　だけ　たべられません'. Like this, you can put 'だけ' after object to make a negative sentence. Does it make a sense?