What kind of signal speed are you dealing with? As traces are brought closer to other traces, the characteristic impedance decreases. That is why I generally aim for an impedance 10% higher than the target impedance. The calculators you can find online ignore surrounding objects and traces, and I have found 10% to be a safe estimate. So design your trace widths for 55 ohms instead of 50.
Another tool I highly recommend is Hyperlynx SI, though it is very expensive. If this is for a one-off board, I recommend the trial version: https://www.mentor.com/pcb/product-eval/hyperlynx-si-virtual-lab
Hyperlynx uses Maxwell's equations to more accurately calculate the effect of surrounding objects on the impedance, and will also give you an idea of how much crosstalk you'll see between two traces. Generally you want to keep traces as far apart as is allowed by the board size, shape, and component layout. As mentioned, guard traces are probably not necessary.
There's really no way to say for sure how far away traces need to be from other traces to avoid crosstalk--It all greatly depends on your layout, signal speed, trace size, etc. Just space them as far apart from one another as you can. Then order a few boards you can use for testing, and make sure they work. Then you can produce more boards if desired.