Can I submit app to app store, developed for iOS using preview release tools?

No. You need to either use the latest version of Xcode 6 (for iOS 8 and earlier) or wait until the GM release of Xcode 7 for iOS9. There's usually an email from Apple telling you when this is available - normally a week or 2 before the public release.

Can I work on my iOS 7+ app using Xcode 7 to add features and prep it for iOS 9, but still submit it to the App Store once per week using Xcode 6?

If it can be successfully built with and submitted by release tools, sure. Just be sure to option-clean firt to remove any fingerprints a beta may have left behind.

You have to build and submit it with a non pre-release xcode version.

Otherwise you will receive a validation error like "The app may have been built or signed with non-compliant or pre-release tools."

Ok. Thanks KMT and spielplatz. Sounds like I can develop full on using Xcode 7 and then when I get a feature ready for release (e.g. 1 week from today) I can simply open the project using the latest version of Xcode 6, do a regular "Clean" followed by an option "Clean Build Folder...", then use Product > Archive to submit my iOS 7-9 app to the App Store even before iOS 9 has been released to the public. This sounds great. I'll try this workflow this week. Thanks.

>submit my iOS 7-9 app

It won't really be anything more than a "tested on iOS 9" app until you build for that using release tools, so plan on another update when that becomes possible. As well, keep in mind that what you're testing on in the mean time can/will change by the time release hits the streets, so don't get too comfortable.

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