Cardboard isn't water-permeable the way landscape fabric is and I've always been taught it's best to use this cardboard method when preparing a new bed, giving it at least a season or two so that the elements break down the cardboard. You shouldn't plant directly through cardboard! As
@Summerbaby said in the last thread, I don't use use landscape fabric, so I can get self sown seedlings! I have used the carboard method just once to prepare a new portion of a garden bed and it takes some time to break down - I started it in the Fall and planted the following Spring and I just don't believe it would have been the right choice for Michael, since he wanted to plant right then and there.
People are mixed about how close to plant plants. Even through my training, I know that there are space requirements, but I plant my plants closer together than recommended anyway, to no detriment that I can see! I may have to divide perennials more often than others who do follow the space recommendations, but it's not been too bad! The only time I follow space requirements, is when I'm planting shrubs, as they aren't easily moved!