Your best option for carpets in low tech tanks are grass types like Sagittaria Subulata, Helanthium Tenellum, Littorella Uniflora or Lilaeopsis Brasiliensis. You could even do a mixed carpet of a few and see which does best, I think mixed carpets look more natural anyway. I think its worth going with the invitro pots for these as they are already in their submersed form and you get loads in each pot.
Cryptocoryne are a great plant for gravel, really big mix of species available now loads of shapes, colours if you push root tabs in at their base too they will do really well. Amazon Swords are an other good candidate but get much bigger so make sure you put these towards the back.
For big stem plants things like Hyrgophila Siamensis 53b, Limnophilla Sessiflora and Cardamine Lyrata are really easy to grow plants and fill out quickly too.
Wills