Qubes not loading without manual selection in boot menu. Is it actually a problem? And few more questions of newbie

[apparatus]

You can install it using the free space on any of your disks the same way you can do it with any other Linux distribution.
But be aware of the possible security risks with multibooting:

Multibooting Qubes

Introduction You should think carefully before dual booting Qubes on your box. Read the guidelines carefully. One problem is that when you dual or multiboot, even if you are using encryption on your Qubes installation, /boot is still unprotected and could be maliciously modified by the other OS, possibly leading to Qubes itself being maliciously modified. The other problem is firmware security - for example the other system could infect the BIOS firmware, which might enable compromise or spyi…