I don't quite get the reason for trimming them. I only trimmed seedlings if they flopped over due to insufficient light. Grown directly under the fluorescent lights, they never really flopped.
Not sure about your question regarding outside. You could be growing them outside. Onions are typically sown as soon as ground can be worked, and onion plants can be planted a month before last average frost. You could really have directly sown the seeds outside if you can keep the patch watered and weeded.
Some references say you can plant them out when they are as thick as No. 2 pencil LEAD -- or about as thick as a toothpick. But they are more susceptible to slug/snail predation at that size. Other references say -- and purchased plants are typically -- as thick AS No. 2 pencils. It's easier to mulch them right away at that size.
Starting them this late, you are not likely to get any real bulbs in the onions, but you can eat them as green onions. Give them lots of high N fertilizer or fish emulsion to get them to grow quickly (this is reminding me that I meant to fertilize mine over the weekend....)
Imafan gave you good references. Dixondale provides very thorough and specific growing information.