I've installed Slack 14.2 on /dev/sda1 (/dev/sda2 is swap) and FreeBSD 12 on /dev/sda3 and lilo is the boot manager.
FreeBSD slices are as follows;
/ on /dev/ada0S3a, swap on /dev/ada0s3e, /var on /dev/ada0s3b, /tmp on /dev/ada0s3d and /usr on /dev/ada0s3f.
I hesitate to install Solaris 10 update 6 after the existing partitions (/dev/sda4 or /dev/ada0s4) lest Solaris deletes or corrupts the Linux/FreeBSD partitions.
Will it be safe to install Solaris here?
You should install on an entirely different main partition.
Please post your entire fdisk list of your partition table.
I would recommend to avoid doing it. If you properly tell Solaris where to install itself, i.e. a free primary partition, existing file systems and data stored on other primary partitions won't be affected. However, Solaris installs its own version of grub, overwriting existing stage 1 boot loaders, so you would need to manually reconfigure Solaris grub to be able to boot existing Linux or FreeBSD's OSes or to chain their boot loaders, which might not be simple.
A much simpler approach would be to install Solaris as a virtualized VM. Solaris 10 works just fine under VirtualBox.