But this is only the space required for the base installation. You might want to add a lot of GNU-Tools, and other stuff. And if you want to share data between Solaris and Linux, this has to happen on the Solaris partitions. You might even think of sharing your home directories between Solaris and Linux. Here's the quick check list.
Some people don t like building a custom kernel.
If, however, you do not want to share data, you are safe with your old kernel. Unfortunately UFS writing is still experimental. Fortunately, the Solaris boot manager is able to chain-boot. And don t forget to run lilo to have lilo in the boot sector of your hard-drive. If you reboot now, you should have a lilo-prompt.