Commercial multi
I have worked in three programs where students obtain their initial Commercial-Instrument in the multi. First off, a lot would depend on the course. In two of the programs, ERAU and Mesa, students were introduced to the single-engine commercial maneuvers before earning their Private multis. They earned their Commercials in the multis. Then, at ERAU, they got back into 172s and earned their Commercial Singles. At FlightSafety, they earned their Private multis and did the remainder of their instrument work and honed their skills to Commercial standards in the multis before earning their Commercial-Instrument in the multis. Getting a Commercial SEL at FSI was an extra course. Of course, most students earned it.
Based on my experience, I like earning initial Commercial certification in multis. The flight test is really the same as the Private multi. At that point, you can get into the right seat, work some more on instruments, and earn your initial CFI as an MEI and CFI-I. I feel that plan gives you the most bang for your multi buck. Then, hop into a simple single to earn your CFI-A.
Both methods work. There are plenty of pilots who've gone on to success by earning their initial Commercial-Instrument in singles and adding multi privileges later.