Yes. The TUFF Products Original 8 In-Line Mag Pouch w/Removable Flap will be able to accommodate 21 round Sig magazines. The correct models for you would be MPN:7068-CBV-1 (Coyote Brown) and MPN:7068-NYV-1 (Black).
Yes it will.
Yes, this product can be used with single stack mags also.
Please reach out to us at techsales@opticsplanet.com and we can provide you recommendations.
These are 10"W x 5.5"T
No, this is for normal pistol magazines (up to 17 rounds for example), it will not fit 30 round magazines.
These would be appropriate options.