I have 6 little seedlings! This is the first time that I have planted rose seed that has germinated (all OP).
When do I pot them up, how tall or how many leaves before potting them up? Any size or kind of pot to plant them in? How do I take care of them once potted up - e.g. still under light, how often to water etc.
You will get much different advice on this. The most important thing to know is that, while young, damage to the initial root tip will almost certainly spell death. I usually wait some weeks before I do my initial transplants because of this (I still try though, but have better success if I have waited). I put them in a small pot first and just upgrade sizes as needed (so they get re-potted several times). Roses are drought tolerant (which is good for me) but can also withstand wet soils. I don’t think you can over water as long as they’re not standing in it.
Keeping the surface of the soil as dry as possible will help you to avoid damp-off. Watering them from the bottom worked well for me when I transplanted to pots.
Do you have a copy of the blue book - Hybridizing for Beginners?
It answers a lot of your questions along with additional information.
Use the order form on this web-site if you would like to order a copy.