I grow all my onions in the garden or in cold frame.

EXTRAs : Just plant them. You can thin them later and use them as scallions; Why waste: I would plant twice as many and thine every other one later for green ionio/scallion usage

Onions love cool weather. keep them under light but COOL. Say 55 - 65 at the most. And transplant them when night lows are better than 32F. Don't worry abot frost or evevn freezin temps.

This my second year doing onions from seed so I am not an expert but I found them to be quite durable and resilient. Therefore, I would keep them growing like that until transplanting into the ground. At that time, trim the greens down to about 3" and separate the miniature onion bulbs' roots. You don't really need to treat them with kid gloves so just pull them apart and place them fairly close together about 9 per square foot. Last year I learned onions are very big feeders requiring a good deal of fertilization. 10-10-10 fertilizer regularly after the plants get established. Also, I learned the onion bulb doesn't really grow underground. It really sits on the soil with only the bottom 15%-20% of the onion and the roots actually in the soil. good luck.

Don't know if this is getting to you too late but Don't put your onions under lights!!!!!
Onions are daylight sensitive, so if you give them too much light they will start to bulb. That would be a bummer if they were only 5 inches tall. They are not like tomatoes, they won't get tall and spindly without lights, you can grow them in the windowsill or better yet, in a cold frame till the soil is ready to transplant (sooner the better).
As far as the clumps, if your soil is loose you can plant them in clumps of 2-3 onions every 6-8 inches. They'll just push each other apart and should all size up nicely.. If you have clay soil, separate them b4 planting and plant them 4-6 inches apart.