So my idea is just to trim and tidy up these grasses (in the photo) with a pair of scissors, would that mess them up?
Cutting them back at this time of year is not advisable. You might try raking out the dead foliage in the sedges (I think that's what the flopping ones are), or using rubber-palmed gloves to pull out the dead foliage. If I cut my sedges back (which I'll do every 3rd year or so, unless they look too ratty) I do it in late winter.
The upright bluish looking plants look like California Gray Rush (Juncas patens). They should be cut to 3" in late fall.
Thanks, so should I cut the other sedges/grasses back to 3 inches in late winter, or does it matter how short or long I keep them?
Most sedges will be evergreen in your zone, so you should only need to eliminate dead foliage.
I can't see what other grasses you have in the bed.
In our climate, these evergreen grasses (and that includes the sedges and reeds) do not get cut back - only groomed to remove dead foliage. Trimming to "tidy up" is not advised - these are as tidy as these guys get. Cutting back only makes them look like they've received a Marine haircut - very artificial looking and tends to stay that way!! Many do not respond well to a cutting back at all....they may even croak.
If you want something tidier, maybe these grasses are not what you need.
How about a 'cutting back compromise'?
You didn't actually want to give them a brush cut, did you?
In my zone (much different than yours), evergreen grasses sometimes come thru the winter with brown, sunburnt bits. Some of these can take a brush cut & others hate that treatment.
Rather than scizzors, I take a sharp knife & tidy up the grasses by pulling handfuls straight up & just runnning the knife blade along - like backhanded teasing. this takes the dead bits off & leaves the grass much more natural looking. The difference between a 'blunt cut' and the 'layered look'