No they do not cross over you can kill and add as additional be careful and the science is different in marine world
Add to the flow is best if you have equipment (filter) to do this,,, no sponges or rings required if LR is used
If Live rock,,, left it do its stuff, if you can get mature water from LFS rather than fresh RO / DI salted that will help and use safe matured LR always
keep testing the water and write it all down look for spikes as that's the cycle,, test same time of day every time also is a good tip