some distro installers will not proceed without installing their own bootloader, how fedora installer works i’m not sure. however it should not be a issue, yes it is recommended to use manjaro’s bootloader for loading other distros.
assuming you have a UEFI setup, what you can do is after installing fedora, boot into manjaro live medium and chroot into manjaro setup and using efibootmgr to alter the bootloader order so that UEFI selects manjaro bootloader over others. refer;
then make sure os-prober is enabled in /etc/default/grub and run
sudo update-grub