If you do, then you'll have an easier time maintaining both of them.

Not sure about versions -- haven't hunted for Tomato for a WRT54GL for a while, but I suppose the "official" v1.28 is a good enough starting point here. There are other derivatives out there that are much more up-to-date, but difficult to find. Eg. the "Toastman" variant.

For the router that already has DD-WRT installed, updating to Tomato is as simple as using the firmware-upgrade menu item.

For a stock Linksys WRT54GL, installing Tomato over the factory firmware probably involves a little extra effort -- I vaguely recall a multi-step process was required. Mr.Google can probably find the info.