30 Liters = 7.9g. With gravel maybe 7.5 - 7g of water. That is a very small tank. I would not put Cories in any tank under 10g. Unless they are Pygmy Corys and you aren't adding other fish except tiny ones.
True Julii Corys are very rare. The LFS probably has Corydoras trilineatus. They get a bit bigger and are commonly mislabeled C julii.
Regardless, both species are too large to keep the recommended 6 or more in such a small tank. Even 3 of them won't fit. They need room to grow and swim and they do best with soft sand. Rounded smooth gravel is ok. Beware, some colored gravels are so cheaply made the color starts flaking off.
Stocking a 7g tank ?
A single male Betta
You can try Ghost Shrimp.
Or
A group of micro Rasboras and Cherry Shrimp
Or
just Cherry Shrimp
A Betta, if that is your choice, will appreciate a live plant or two. Try Anubias as they can rest on the leaves. Or Java Fern. Both are not "planted". Tie or glue them to a piece of safe rock or driftwood.
Betta tanks should not have ANY plastic plants. Sharp edges can tatter the fins. Silk plants are best if you want to use fake. No sharp edged decor either.
Bright gravel competes with your fish IMHO. Later if you want to change, consider white or black sand. Makes a better frame for your fish to my eyes.
Enjoy your tank !
This was my last Betta boy in his 10g palace.
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