This is one of those "be careful what you wished for situations". You wanted the Vals to grow, and it sounds like they did in a big way.
When it comes to rearranging, Vals are incredibly hardy. The only problem I see is that sometimes the a few leaves will pull off easily when you are moving them around, but just as easily as they pull off, they'll grow back.
When the runner plants start to have leaves that are 2-3" long, you can cut them off from the mother plant, and then move them/reposition however you'd like. At this point, you can even take them out and trade them/throw them away if the Vals are starting to outgrow the area you'd like them to be in your tank.
And as said above, they'll be fine in the lower light situation. If anything, it may help you from having to prune so often.