I wanna cancel growing two of my plant sylestis to make room for my spring storm zolnixis, since i dont know if there will be enough time to grow my zolnixis by the time these two are done, i dont even care if i dont get the seeds back because i dont care about these two plants at all

As far as I'm aware, you can't.

If you take frequent care of the plants you want gone, then you could possibly get them done by the 18/19th, so I don't think you'll have enough time to plant & finish them unless you get them done today or tomorrow. (As you can grow a pet in ~3 days if you take care of them enough.)

You can always sell stuff/pets for eggs in order to buy pots.
Or maybe you can ask Krin about removing them for you? (That's if he can.)

Edit: Krin ended up extending fest time to the 27th, so you now have time to grow your plants.