Change order a icons in system tray widget in KDE?

No, this is currently not possible. A proposal that implemented this ability was rejected by the maintainer because of the already fragile code base and added complexity: https://phabricator.kde.org/D11233

I am not aware of an alternative widget that allows you to make your own system tray, since the system tray widget is closely bound to the C++ backend, which makes it harder to theme/customize this freely (see https://zren.github.io/2018/11/17/exploring-plasmas-systray-widget#this-is-just-for-fun).