I thought a simple explanation about these bonus buildings was in order, in case anyone has any questions. The math itself is simple. All a bonus building does is raise the gold-per-minute stat of one type, say Plant, by 20%, and then lower it's opposite, which is in this case Ice, by 20%. Example: LV 1 plant = 17 g/min x 1.20 = 20.4 (I'm assuming it rounds to the nearest whole, haven't tested) LV 1 cold = 4 g/min x 0.80 = 3 g/min
Now, here's the interesting part. This effect only happens on the island you place the bonus building upon. Ergo, if you want to keep your ice dragons happy, but you want to give a bonus to both your fire and plant types (their bonus buildings both knock down ice by 20% each), then put your ice habitat on a separate island from your fire and plant ones, and you'll have happy dragons all around!
If you can't get an order where all dragons are happy, just remember that the dragons with the lowest gold per minute rate will be taking the least hit for the 20% reduction, so they should be the ones to 'sacrifice'. As in the example above, you can see we gain 3 gold per minute but then lose 1, giving a net gain of 2 gold per minute, which is still an increase.