Hello Tom, nice to meet you!
You are perfectly right, in Spanish, you can add the adjective before or after the substantive.
But is true that the general rule says is better to put the adjective after the substantive.
You can perfectly add the adjective before the substantive and it will be totally correct. But putting the adjective before the substantive gives a more poetic and subjective feeling. In poetry, adjectives usually go before the substantive because it sounds better and more poetic since it's unusual.
Therefore, "flor bonita" is what people would normally say. And "bonita flor" is what you probably would find in a poem.
Also, you have to be aware of one thing: putting the adjective before the substantive can change its meaning. Not always but in some cases.
For example:
- gran chicac-> the girl is great, it has a great personality, the adjective is a personal point.
- chica grande-> the girl is big, her appearance is big, the adjective is a physical point.
Another example:
- mi antigua casa-> the house where I was (before moving, etc.).
- una casa antigua-> a house that is old, ancient, etc.
Hope it helps!
Please ask me if you need more help!