Hi Jonas,
I've hit the same problem that you did, but to solve it I signed the driver that Juniper provided for the SRX550. This allows you to install the driver without needing to change your driver signing settings in Windows 10.
http://weaklogic.com/wp/juniper-srx300-and-srx340-usb-driver/
The only file added was 'jnprvcp.cat' from the driver provided by Juniper.
Hopefully it's useful to you, or to someone else.
Cheers,
Niels