Sure, a 7V adapter would work to slow the fans down. Just be sure to use one adapter on each fan. No splitter after the adapter.
Just for reference, it's fine to connect multiple fans to a single MB fan header using splitters or hubs. You just need to check the rating of each fan and make sure the total is less than the rating for your MB's fan headers. They are probably good for 1.5 to 2 amps (18 to 24 watts), but to be on the safe side, staying under 1 amp (12 watts) should be fine. Check your fans, but the Nocturas I have are rated in the 1 to 1.5 watt range. Running multiples is no problem.