That is probably not going to work. The days are so short the end of April that the plants will not get much vegetative growth and start budding before they have any real size to them--probably when they are 4-5 weeks old. But, because the days are getting longer, they will try to go back to vegetative growth as we approach the solstice. This causes a show down in growth while they change from flowering hormones to vegging hormones. This causes weird growth with the plant throwing out single unserrated leaves. Eventually (2-3 weeks) it starts growing normal leaves again. Then 6 weeks or so after the solstice when the days start getting shorter again, they will go back to flowering. This changing from vegging to flowering to vegging to flowering is going to hurt their growth and budding. You will have wasted weeks where the plant is trying to figure out whether it should be vegging or flowering. There simply are not enough hours of sunlight to get good veggy growth. If you can start them inside and put them out like the end of May or so, it would probably be better.
I am not quite understanding your first point in your above post? If they outgrow their container, transplant them into bigger pots.
I generally use either seed starting mix or rapid rooters to start plants.
When temperatures get below 60, plants pretty much don't grow.