I have grown Celosia argentea var. plumosa from seed for the second year now. They need regular soil like potting soil or garden soil because they are supposed to grow tall and need to anchor their roots well. If the soil is too light, like the one in the question, it won't provide stability.
When you notice the seeds have germinated move them to a sunny location, otherwise they will become etiolated - long and thin stems that eventually will bend even if their roots will be well developed.
I don't know what the effects of garlic spray is, but I haven't sprayed them with anything but tap water and they did great. Usually, plants that grow unpampered don't need anything except soil, sun and water, so don't bother with fancy rituals found on the internet.
If pouring water makes them fall, use a water pulverizer instead. Remove the plants if they are clumped together and keep only one in every 2 inches (5 cm), otherwise they will grow very slowly and as they accumulate sunny days and become older, they will produce small flowers even if the plants are 3 inches tall (8 cm).
I have kept my seedlings in a room with the south wall made entirely of glass with the seedlings on the floor, right next to the glass. The ceiling was made of glass also, and the plants grew well, so don't worry if you grow them in full sun. However, if you grow them in indirect light or in a shaded spot and then you move them to a sunny spot, make sure you gradually expose them to more light, a few more hours every day until they can take a full day of bright sun.