Staging 18mm to 13mm is preferable, partly because of concerns about nozzle sizes already mentioned, also because generally you want the more powerful motor in the booster as it has to lift the heaviest load.
Gap staging is, in my experience at least, more reliable than having the motors jammed together.
There used to be a nice kit called MIRV Gryphon, which involved a cluster of 13mm boosters in a cluster, each staging a separate 13mm sustainer. The A10-0T went out of production; meanwhile, for entirely different reasons, I didn't get to fly anything for a while and went insane from BP deprivation. The result was MIRV Gatling - six 18mm boosters in a cluster, each staging a separate 13mm sustainer. Everything lit, the booster definitely came back down, and at least one of the sustainers definitely landed as well because it was found some weeks later.