Some Petcos have folks who know what they're talking about, but a great many do not. (My local one has a fish dept. manager who _does_ know what he's talking about, but he keeps trying to sell me things that are bad for my tank anyway.) Generally, it's a good idea to take Petco's advice with a grain of Instant Ocean.
I haven't looked into them too deeply, but chocolate chip stars are generally regarded as _not_ reef-safe. "Classic" sea stars, in general, are difficult, and not a creature for a new setup - or a small one. Serpent stars or brittle stars are generally a better choice.
Your "mushroom" looks a great deal like my maxi-mini carpet anemones. They're a fairly hardy (and fairly sticky!) anemone that comes in a lot of colors, ranging from greens to yellows, reds, pinks, purples and even blue, with or without stripes and patterning. They can be dangerous to small fish who make the mistake of touching them.
~Bruce