I waited til all my seedlings were ready before transplanting, and then I waited til my holidays started so I could keep an eye on them. In hindsight some of them could have been transplanted 2 weeks ago, just wasn't prepared to risk damaging my choc bhut jolokia which was still on its first 2 leaves.
I waited until most of the Chocolate Trinidad Scorpion seedlings had 4 true leaves. The roots were more entangled than I would prefer. I have managed previously to only cause minor damage by carefully separating the seedlings.
I do wonder if seedling mix has enough stuff in it for a plant to develop, seems like it is designed to be light, and easy for roots to start and grow. It seems to dry out too fast imo. I really like the idea of starting in 10cm pots. I'm currently thinking if it is worth having a lower layer of good quality soil topped with seedling mix to start my plants.
The Debco Seed Raising Mix I use contains two months worth of slow release fertiliser. I'm growing all of my season starts in this mix bar the Brazilian Starfish and Goat's Weed plants. The more advanced plants in 3L and ~5L pods containing Debco Seed Raising Mix. There was enough nutrients in the mix for the plants up to when they needed transplanting from the 3 L pots.
The catch with using two layers of potting mix they may split when you perform a transplant. If you see evidence of a nutrient deficiency then you may be better off feeding the young plants with a weak solution of soluble fertiliser (say quarter of the recommended ratio).
The plants have performed well enough in the mix I mentioned that I'm considering using it in their ultimate pots. A couple of things that is holding me back at the moment is the fact that all my 25 cm / 8.5 L pots are black and space consideration.
I'm testing out Debco Propagation Mix on my over wintered plants to see if it's not as packed with Nitrogen as some of the other mixes. The Fatalii and Seven Not blooms have slowed down but without a control for the test I can't draw a conclusion just from that. There are some segments on the Fatalii plant that have increased internodes. I seem to be over the Nitrogen bump judging by the compactness of the new growth.
My issue with having 2 or more seedlings in a 10cm pot is that I need to at some point pull them apart, and I'm not real keen on snapping root systems in half. Part of the point for me having them start in a 10cm pot would be that I could transplant the whole unit into a bigger pot(30cm) without having to pull out the whole root system which would help keep the plant stable.
I prefer to keep the roots together where I can. My plants though seem to benefit from an intermediary pot size. Going from 0.5 L to 3 L to the final pot size is working for me. One of my goals was to fit most of the plants on a single shelf in three rows to allow for exposure to late winter and spring sunlight. If you have the space, the pots and the potting mix then I say go nuts with 10 cm pots.
I'm yet to use a soil that doesn't come with these gnats your talking about. My seedling mix had them too. I have a bottle of neem ready for my plants. I use something between 2.5ml and 3.5ml per 500ml of water for my older plants and I added about 1/4 of this mix to my seedling water spray which I have in an old aerogard bottle. It did the trick seems I only needed a small amount of neem to piss off the gnats (if that is what they were). Once I stopped using the neem they came back.
I haven't noticed any gnats in the Debco Seed Raising Mix after going through 5 bags of it. My first bag of seed raising mix was an Amgrow branded one that came with bonus spring-tails. Those critters didn't seem as bad as gnats though to the seedlings. There is sometimes an issue with how the shop you buy the bag stores them.
I have killed gnats before in pots immersing the pot in a weak solution of Malathon. The idea is to get the water level above the level of the soil. All of the little bastards will find their way to the surface and die. In the case of my Aji Amarillo in the gnat infested mix it seems some gnat predators have struck a balance finally.