Install OSX 10.5 on a 2nd system then upgrade

I have just bought a second hand 20' imac next to my 24' I already own. Now on this second hand i did not get the original 10.5 install CD's.

I have these CD's from my 24' . Can i install these on the 20' as well or will i run into trouble?

I just need to install 10.5 once as I have bought a family upgrade pack to 10.6 snow leopard.

Just wanna start fresh with the second hand and format it, put on 10.5 and then upgrade to 10.6.

Hope this is clear and just need some insight on if i will run into trouble with this as it seems a bit silly to spent a lot o money to just buy a standalone 10.5 CD for a one time install.

Thanks in advance for the answers