you will have room for the heaters, we have one in the same chambers as the skimmer pump and the other in the very next divider before the bioballs.
personally, while im not a fan of bioballs, i would leave them. we still run them and it has not caused us any issues, also, when you need to, they are easier to clean out (dont clean them all at once either).
shame about the apistasia, sometimes hard to manage. we have it in ours, however my copperband is limiting it to our sump only.
where are you located?