I am building gentoo through a fully installed Ubuntu system. My swap partition is in use by the Ubuntu system so when I use the mkswap command I get that the partition is already in use. I expected this but I was wondering if postponing the mount of swap until after I have booted into the gentoo system would cause any problems. Am I going to have to edit a bunch of files if I mount my swap later?? Also, I didn't see any manuals mentioning building a gentoo system from a fully installed system, is that going to cause any problems? This is my first gentoo installation, so I'm not really experienced with gentoo. I didn't think that building from a full install would be any different than a live cd other than more resources are already in use.

I'm not actually having problems with install. I just want to make sure I wont. Thanks.

No, if you don't run out of physical memory, then your machine won't give two hoots about the state of the swap. and adding it back later is just a case of a line in fstab and nothing more at all.

And you *really* don't want to do that mkswap at all.
Because if you do the UUID will change and Ubuntu won't boot. One of the perils of using UUID.

Thank you both for the VERY quick response. And thanks for the warning about the mkswap cmd. I didn't think about that. Also, I have plenty of ram so I should be fine (8gb). So all I will have to do is edit the fstab later right, I dont have to use the swapon cmd or anything right??

no, swapon is just the direct equivalent for a swap partition to mount for a filesystem, all handled on boot.