Heros are best in singles or in groups of four at minimum but preferably six.
I have been working with Heros for about sixteen years, breeding every described (except H. spurius) and undescribed species I can get my hands on. They are one of the most versatile and compatible SA cichlids you can keep. However, being cichlids, they do have moderate levels of conspecific aggression. This is especially bad in a few instances:
- During "teenage years" at 3 - 4".
- Introducing new Heros into the territory of an existing Heros
- Keeping them in tanks that are too small
- Trying to keep pairs alone with no other fish
I've found that, like discus, trying to keep 2 or 3 Heros together is usually a recipe for disaster. One dominant fish will usually emerge and mercilessly harass the other two, or two will pair off and gang up on the third. Getting at least four will balance out their aggression a bit and spread it around so that no single fish gets too beat up.
I like 75 gallon tanks as grow-outs. I usually keep 4 - 6 in a tank that size until they're maybe 4" or so, then upgrade them into my 6' community tanks. On rare occasion you might be able to keep a small group in a 75 for life but that would depend on their adult size and their temperament. For instance, H. severus and H. liberifer easily get to 11" or more, and a 75 would be too small for them. My giant H. severus pairs took over 220 gallon tanks and destroyed their conspecifics, shredding fins and scales. But something like rotkeil which are often smaller may do okay in a tank that size.
Personally, I think you can add three more little ones with your current fish and that size tank will be adequate for at least 6 - 8 months as they grow. Just keep up with water changes. Then you can move them all to your 300 gallon if you have space.
Another tip: if you want relative peace among them in the big tank, keep all males. Having a mix of males and females will usually lead to pairing which could lead to war in your tank as the pair defends a territory, and I've learned more than once that several single females will typically either pair together and wreak havoc like a male/female pairing or they'll hate one another and fight constantly.