You can put the heaters in the sump. Most ferns, like java fern, and anubias don't need to be planted in substrate. Most people tie the anubias to rocks until they attach and the ferns usually stick to driftwood (and never come off!!). Other plants, like swords, do well in sand provided you use root tabs with them. Based on your 90g tank, you could probably have 8 fish, without accounting for your sump. Some people would say you could have more with the extra volume of water, some would say that less is more and your fish would be healthier because the water quality would be better/you'd have to do less water changes.