Hello Garnick,
It technically doesn't matter, but I would remove Font Book before installing your new font manager. See the bottom of section 6 in my article for the full instructions for removing Font Book, clearing its database, and clearing all current font cache data.
Yes. Suitcase will automatically add all fonts in the System and root Library folders to its list. You don't need to do anything special. It's ready to go from there. Up to you, but the first thing I'd do in Suitcase's preferences is to turn off the Vault. Change it to activate fonts 'in place".
To me, "heavy use" is having to manage numerous sets of client fonts each day, turning them on and off as required for each project. You can of course do this with Font Book, but the more fonts you have to handle, the more likely it is to choke.
By following the minimum font lists in my article, that's as lean 'n' mean as you can get the OS without crippling the OS X supplied apps, or making the web unusable. Be sure to pay close attention to both parts of section one. The first part is the minimum list for the System folder, and the second is the minimum list for the root Library folder.