Seymour was a role I've auditioned for many times. Don't play him as a cartoon. But you might want to emphasize Seymour's enthusiasm and need to please other people. I don't know how much material you are allowed to sing, but remember Seymour has some high rock notes during the "Meek Shall Inherit" sequence. You want to pick a song that will illustrate you can do that.
Oddly enough, I used to have good luck singing Godspell at "Little Shop" auditions. I'd sing "All For The Best" as a comic song and "All Good Gifts" as the dramatic piece which could show off the high rock notes.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.