Once Grub (the bootloader) is installed once (usually done when installing any distro first) it can multiboot between whatever other distros you may wish to install in paralel. The tricky thing about having multiple distros though, is that each might have a different version for Grub in its repositories, which may cause situations during upgrades where each distro reinstalls and reconfigures Grub according to the package and configuration available to it.
The simplest way to avoid this would be to let the distro with the most up-to-date Grub package in its repository to install the bootloader and have all upgrades to it be done as part of its update cycle, where the rest of the distros won't have the Grub package installed at all. In such a case the other distros would depend on the Grub update script from the "main" distro to have their boot entries added and updated, which is usually done automatically.
So in a nutshell, what you will end up with is a single bootloader that is managed by a single distro and allows multibooting between all the other ones installed. The order of installation shouldn't matter as long as you make sure Grub gets auto-configured properly (which iirc should be done automatically and seamlessly anyway).