First
to ATP ,
not sure I am the best one to give advice this is only my 2nd year planting seeds of any kinds not just coleus , but I have read after the first or second set of true leaves (not counting the first set of germination leaves) it is ok to transplant. Personally I just usually play it ear , if they are overcrowding or I think they need a deeper container , I usually dig young seedlings out gently with a plastic spoon , place them in a plastic or glass dish and gently mist off any dirt with a misting squirt bottle ,
gently separate tangled seedlings with ( I use a bobby pin ) but any small tweezers or instrument , poke a big enough hole in the moist soil of new container / pot / cup for the new seedling and place it in , once you are happy with it's placement , gently push in surrounding soil until you are happy that it is supported firmly enough and lastly maybe a final misting squirt or two to seal the deal.
