in a 60"x18" you would be fine with a few species of bichir,and an ornate would be fine for quite some time as they aren't particularly fast growers compared to some others,as already mentioned a del would be good as they max out at around 14",if you are goimg to keep a sen with some smaller fish then watch out for a size difference as senegals are fairly fast growers and can eat suprisinly big fish,but some seem more likely to do this than others ! good luck Paul