You should first tell us what you want to do!

When you install a kernel (step (1) you mentioned above) it is placed in a special partition so that the boot loader (NOLO) will run it.

U-boot acts like a kernel, i.e. it is stored on the kernel partition so that NOLO runs it. U-Boot can then run an attached (embedded) kernel (usually the default Nokia kernel) or another kernel of your choice, being installed somewhere else (another partition, Mydocs, sdcard, whatever).

For completeness:
Multiboot is for all I (we) care deprecated. It basically replaces the installed kernel (i.e. in the kernel partition) with whatever kernel you choose when booting. I've never actually used it, and I don't think there's any valid use-case for it. U-boot does that and much more and much more reliably.

Then there's bootmenu and backupmenu. They are not kernels but they run very early (first thing after the kernel has done its initialization). You can use bootmenu and/or backupmenu independently of which kernel you boot (Nokia kernel, kernel power, or U-boot with some kernel).

In short: if you now install kernel-power it will install itself in the kernel partition (which for you is OK because you don't want/need/know-about U-boot). If you have multiboot it will continue to appear but I'd advise you to remove it. For this, please ask again (or make a new thread, or search here). I believe you only need to replace /sbin/preinit but as I said I've never installed/used/uninstalled it.

Good luck.

PS: Before you do anything, make sure you have a working set-up for flashing the N900. That means the firmware, the flasher and a computer than can actually work with that. If in doubt, ask first.