Many if not most employment opportunities require Multi-engine experience. The Commecial Pilot Licence preparation can overlap with an Initial Multi-engine Rating preparation.
However, if deciding to combine the tests for a Commercial Pilot Licence and Multi-engine Rating it may be prudent to allow an additional five hours 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 a designated examiner is required to complete the Multi-engine Rating. Bear in mind the initial Multi-engine rating requires 70 hours pilot-in-command before being tested.