If I were to say do a Lost City IL run. Could I use plants from let's say pirate seas for it?

Yes

As long as you start from a fresh file and follow our rule page, you're good. Just keep in mind that each "outside" level you complete will probably take 2-3 minutes, so be sure to evaluate whether the detour is worth it if you're going for a top time. Good luck, and let us know if you have any questions or need any tips!