Plants, given they're receiving all the proper conditions -- lightning, co2, nutrients from substrate and water column, and trace elements from water changes can generally handle a ton of trimming unless they're slow growing.
The general rule of thumb I use is..if my undergrowth is starting to die, I cut everything stopping it from receiving light. The aquascaping ''experts'' you see in the top ranks use methods I'm not familiar with but seem to resemble shrubbery pruning. lol In some cases the most aesthetically appealing tanks don't really seem all that 'natural' to me, but they are indeed pleasing to the eye.
One of the easiest tricks I've noticed that aquascapers use is diagonal lines and grouping. They keep most species of the same kind grouped together (which is somewhat natural). They also use a natural sloping of plants. IE, something tall growing at the back, and as you work your way to the foreground you'll come into mid-level plants and finally things like dwarf hairgrass/mosses/dwarf baby tears/etc happening back-front and right-left or left-right. The diagonal lines created by this create a more ''dynamic perspective" which is a trick photographers use in landscapes often. There's also usually a decent amount of visible substrate to the center or one side to create contrast to all the plants. Or, sometimes its just completely grown over with carpet plantings. Many people also like to use a nice piece of driftwood to break up the plantings a bit and create some more contrast. If you have one in there, aplogies...its hard to see past the first inch of plants. lol
As long as you know what plants you have there's a great deal of readily available information on the web regarding each species and propogation. A lot of plants propogate by trimming, which is handy. Though it doesn't look like you need much propogation. :)
You have some nice looking growth in there.