My recommendation is you’ll need to run Xcode 9 on your mini and then get one used Mac for every other contributor. They can connect to the build server on iOS 9 and either use Xcode themselves or Screen Share as the second account each Mac allows fully supported.
You will waste far more effort, suffer low quality code and delay working around tooling issues than you’ll gain by not funding your development team with tools that work. Buy broken Mac at scrap prices and fix them up or cook dinner and trade labor with someone that can repair Macs. Do anything you can think of to get your team Xcode running on macOS.
It’s like training dentists remotely with no tools but books and then hoping they perform as part of a team still without tools. Your customers will not be fooled when they see the results. Worse, if your iOS app is going to the App Store, you’re competing against every team that had funding for 4 years college, the best training and tools.