How to determine the sizes/arrangements of supply pipes?
Usually if you have multiple users from the same line, that line should be bigger in order to avoid unbalancing

EG: you have a toilet a sink and a shower, you should run 3/4 pipe across all and connect each user with a 1/2 manifold

Another solution, if you can't replace existing with bigger pipes, is to use 'automatic pressure balancer' on the lines serving all, so if a big pressure drop is found on cold water, the same pressure drop will be applied to cold water so, at each end, the pressure isn't different.