is it possible to plant those plants that they sell in the LFS where the ends are tied to a cotton with a metal clip and have no roots?

Yes. Just remove the metal thingy with the sponge and plant it and it should grow roots into the sand/soil/fertilizer combo thingy I assume you already have set up in the tank...

I've seen people who stick the whole thing with the aluminium and the sponge too but how well that works I don't know...

Yups, it will grow. Gotta untie the metal clip and place them in sand or fine gravel. It will take up roots in 2 weeks. Or you can tie the ends of the plants to a small pebble so it won't float up.

it will grow. the metal clip is just to keep the plant from floating. after the roots appear, you can remove the clip and let the gravel hold the plant by placing the roots into the gravel.

FYI, and not all plant that sell in decent LFS,eg. those in jln pasar, are water plant.

buy it from planted tank specialist shop.

what about the gravel below? what size of gravel should be optimum for the plants to grip and does not cloud the water?

If you hv the budget, try to get the ADA soil. Price is depend on yr tank size. Bigger tank need more...of coz.

The budget type of soil is holland sand. most of the lfs selling it. Just tell them you want..."ho lan sah" in Cantonese.

To make the plants grow, you need base fertilizer and some plants need liquid fertilizer. Beside that, lighting and CO2 (carbon dioxide) needed for plants to run the photosynthesis.