Many if not most employment opportunities require Multi-Engine experience. The Commercial Pilot License preparation can overlap with an initial Mult Engine Rating preparation.
However, if deciding to combine the tests for a Commercial Pilot License and Multi-Engine Rating it may be prudent to allow an additional five hours of preparation time. Many, if not most candidates, prefer this approach, although the final choice to test on a single or multi-engine aircraft is left to the test candidate.
The initial instrument rating test can also be on a multi-Engine aircraft. A test with designated examiner is required to complete the Multi-Engine Rating. Bear in mind the Initial Multi-Engine rating requires 100 hours pilot in command before being evaluated.