I would plant the three larger ones in their own pots (same size as they are in now, no larger). The thinner ones are growing like they are from nothing you have done.

These are labeled "RCN" and having the grow wider and not tall, is a good thing. They look very nice. Fertilize with not too high a nitrogen and with micro & macro nutrients within. Numbers similar to 9-3-6 at 1/2 strength when you do fertilize.

I think your seedlings are doing very well for you. I feel you will see a difference with the leaves and growth will be good once potted individually.

Rick

Nice seedlings. Dont worry they become dprmant off and on and recover on their own in the summer. I planted mine a day later to yours and mine have all turned out spindly types.Adeniums are slow to grow and test your patience....but are reseilient too. You can do as Rick suggests and maybe they will get over their stagnant phase once in new pots