Help with clownfish nem tank

[ponokareefer]

You should have enough room for a BTA and enough light. If it does decide to move around at all, it won't have far to go to get to a powerhead though. You might want to consider a larger return pump from the other system and scrap the powerheads. The spray bar should spread the flow out nicely.
The clowns will have enough room in a 15 gallon if it is a smaller type of clown.
Your total system volume of 70 gallons should keep nitrates down as well.