There are a few issues here. Starting with your initial question on the swords: The "runners" I will assume are inflorescences (commonly, flower stalks), since you seem to have one of the "Amazon" swords (Echinodorus species) as opposed to the chain swords in Helanthium. These will have been "prompted" by the plants before you got them. When grown submersed (the parent plant I mean) flowers seldom develop but adventitious plants do.
As for the size of the parent plant, this depends upon several factors including the variant of the species, the lighting, the nutrients, and water parameters to a lesser degree. The "Amazon sword" species is E. grisebachii which has several forms that used to be classified as distinct species (E. bleherae, E. amazonicus, A. parviflorus and E. grisebachii) until Lehtonen (2006) did his phylogenetic analysis and established there is only one true species with various "forms." I won't go into all that. But plant height is related to light and nutrients.
I agree with those recommending substrate fertilization. Seachem's Flourish Tabs are what I use; the API tab is not as good. Liquid fertilization is also important because some nutrients are primarily taken up via the leaves. A comprehensive liquid like Flourish Comprehensive Supplement works here. The API product you mentioned only contains potassium and iron, so the other 15 nutrients are missing. You can grow swords with just the liquid, but adding the substrate tabs does make quite a difference.
Light. This single tube can work but only with a good tube. I have a 29g among my several tanks, and this plant does well with a Life-Glo T8 tube but nothing else that I've tried. The ZooMed UltraSun is another that will work. Either of these will give you low-moderate lighting. There are no T8 tubes that will improve on this.
Increasing duration will not compensate for intensity being too minimum. And lights 24/7 are bad for plants and fish. Both need darkness for a few hours or they will have real problems.
I doubt very much that Cabomba will last, as another member mentioned. I can't grow this in my tanks, with moderate light. But your swords will settle in and be healthy with the suggested light and fertilizers.
Byron.