The general rule is simply that you may only have 1 animal in your deck. Nothing prevents you from having a single Worker bee as your animal. I have no idea why you'd do that, but it's totally possible.

When paying with Mana from your Flowers, simply release that Vegetal Mana in the smaller Mana pile.

A Flower spell is any spell with Flower in its name, or any spell with "Transmutable: Flower".

When a spell mentions "Your Animal" while you control multiple Animals, you may interpret the spell as mentioning "One of your Animals".