Yes you can multisow Kelsae if you want them less enormous, I grew two from sowing three seeds. Your shallot spacings sound fine for sowing 4 to 5 seeds per module.
Last spring I germinated onions over a hotbed but the greenhouse was very cold and they were fine, so gentle bottom heat works well, and they came off the hotbed after a month. Any heat given is most worth it in the first month, after that the plants are happy to grow slowly in cooler conditions and last year endured some frosts in the greenhouse before planting – with few bolters in Red Baron.