I have found stem plants are very helpful for providing new plants quickly.
Stem plants.
Ludwigia repens- Grows nice brown leaves with pink under the leaves in enough light. They will just be darkish green under lower light
Sunset hygrophila- Has nice vein patterns on the leaves with ultra light green leaves that will redden up with higher light. I supposedly have low light but manage to get some pink tips from this plant.
Pennywort- Has nicely colored light green leaves round in shape. It will also respond well to trimming and replanting of the tips. Moneywort is a similar plant.
These are just three stem plants i have that suit low light, grow quickly and can be trimmed at the tip and the tips replanted. When you trim the tips of most stem plants they will often send out two new tips from the joints between the leaf and stem but i would read up on the variety to make sure. You may also want to try water sprite or water wisteria which look similar and have strane oddly shaped leaves but each type will reproduce differently. I have tried growing neither but have read that they will reproduce incredibly quickly and tend to overtake a tank. These two plants may also hog the nutrients that other plants need which might not be ideal.
Rosette plants
Dwarf chain sword will also adapt quickly to many conditions and when it gets established (2-7 weeks) it sends out runners that will grow many plantlets along the runner. They will eventually get to a point where you need to throw it away or sell new plants.
Crypts- I have four varieties. Brown Wendtii (has grown tall quickly and has gone a nice brown for me in low light), Lucens (seems to have stayed short but spread out into bushy groupings which may include a lot of runners I have not pulled it up to check), Red undulata (slowest growing for me and also seems to have gotten bushy before trying to grow tall), Balansae (Only new but grows tall fairly quickly. Crypts are great plants and no doubt i will add more variety over time but they are slow growing and will also require root tabs. If you want plants that can be split quickly then crypts might not be ideal at this time.
Vllisneria- Might grow too tall for both the 3g and 10g but also propagates by runners. I was given one bit of Vallisneria that is healthy but has not sent out runners so far. I'm not sure if this is due to my conditions or if the plant just takes a while to expand, either way it was just added because i was given a sample so i'll see if i can grow enough to make it a feature of it eventually.
Anubias- There are loads of Anubias varieties and they are all slow growing, but when they expand enough you can trim off a section of the tip that has roots and a rhizome formed and it will give you a healthy second plant. The great thing about Anubias is that it can be tied to rocks or driftwood and the roots will attach and form an attractive latticework to cover the rock/wood over time.