you could keep a pair of os in that tank by themselves, just remember to keep up lots of wcs. 2 jds and 2 os would need at least 180g imo. if you ever get jds remember that the electric blues(if thats what you mean by blue) are much more sensitive and harder to keep then regulars, not to mention much more expensive. you might be able to do an o and a jd only im not sure tho. remember if you get two os make sure you get a male and female. you could also do an o, some silver dollars and possibly a severum.