If I only have a few trees, will furniture always spawn in them?
Solution 1:
Ideally you want 17 regular trees in town to maximize harvesting of bells and furniture.
10 will have bells. 5 usually have bees. 2 will have furniture. If you have fewer than 17 the spawn balance is out of whack.
There is no issue with planting an easily-shaken cluster of trees. I would suggest making sure they are planted in a way that will allow them to grow.
Are there any spacing rules for trees?
This is a great guide on how to (and how not to) plant.