If installing the 32-bit / x86 build of wacup (i.e. the version from
https://getwacup.com/preview/) & don't change the install settings then you should already be getting the winamp modern skin as long as the installer is able to pull down the skin files needed & either having chosen it from the skin selection prompt on the first install of wacup or by going to the skins submenu or skins preference page to select it.
If you installed the 64-bit / x64 build of wacup then per the notes on the download page & in the installer it can only support classic / 2.x style skins (which is what the left part of the screenshot looks like with it being the 'classified' version of the winamp modern skin (on the right). This is where you need to use the 32-bit / x86 build of wacup & just getting & running it's installer should get what's needed without having to uninstall as they can share the same settings folder to avoid losing anything that might've already been done (e.g. local library imports &/or custom metadata having been entered into it).
As for the sizing of things, wacup tells Windows to not try to apply dpi scaling so if using a classic / 2.x style of skin then you've got 1x/2x/3x scaling modes. If using a modern / winamp3 style of skin (as the shown 'winamp modern' skin falls under) then you can change the window scaling either from the main right-click -> window settings -> scaling menu or by going to preferences -> skins -> modern skins & changing the scaling option from there.
The final thing about dual instances & things going to a different one, firstly you can run multiple wacup instances using the new instance (via ctrl+alt+n) option & following the prompt to make it duplicate the settings folder to allow for a better dual instance setup.
If you do want to run wacup + winamp at the same time then you're going to have to change the window class that wacup reports for plug-in / external tool compatibility to be different from what winamp reports so the running winamp instance isn't then going to know about the wacup one.
That can be changed by going to preferences -> advanced -> portable mode -> use the open paths.ini editor, change the 'wacup winamp classname' to something (e.g.
WACUP not Winamp), save & then restart the wacup instance. That will then make the winamp instance not find the wacup instance & hand off things to whatever is found to be the first 'winamp' instance which is how external file loading works when the option to prevent multiple instances in either program is enabled. It can alternatively be done via wacup command-line options when applied to the shortcut used to start it which is also something that can be used to run dual wacup instances against different settings folders without having to manually do the new instance action mentioned earlier.
