How are you planning to undervolt your fans? Are you set on a 7V undervolt for any particular reason or have you just settled on that particular undervolt as likely to make the Scythe fans quieter?
Please post your entire hardware configuration. There may be components in there that are noisier than any particular fan at 7V, making your question somewhat moot.
Answering your question:
Slipstream 800rpm @ 7V is <18 dBA@1m, 540 RPM, 18 CFM
Slipstream 1200rpm @ 7V is 21 dBA@1m, 890 RPM, 30 CFM
S-flex F 1600rpm @ 7V is 7V ~20 dBA@1m, 890 RPM, 23 CFM
S-flex E 1200rpm @ 7V is <19 dBA@1m, 670 RPM, 18 CFM
(Source: SPCR Fan reviews # 2 and # 5 from the main page-> Fans section)
So the Slipstream 1200rpm moves the most air but is also the loudest at 7V. However, the 21 dBA noise is below the 23.6 dBA quoted for the 200mm fan on low (although all of these fans will add up to louder than that). Is your system going to be so hot that you will need all of these fans?
Have you considered using software like Speedfan or a hardware fan controller (or even a few Zalman fanmates) to get the best possible result noise-wise for your system?
*Whew* Sorry for all the random questions. You've probably considered a lot of them if you've read some of the contents of this site in reasonable detail...
I assume you have already bought the Antec 900, as the Antec P182 is easier to quiet (although not as good at cooling). I've built my gaming rig into a P182 (see my signature below) with what were at the time some reasonably beefy specs and it is reasonably quiet.
Hope some of this helps!
Main PC (E5200, G31, Lian Li Q07) | Gaming PC (E6850, X38, 5870 Vapor-X, P182) | HTPC (4850e, 780G, 3450, NSK2480B)