Hey now dont beat yourself up over this ... I could tell you stories that would make you cringe ... You are new and learning 99.999999% of us have made the same mistakes in one way or another . Here is one that I will share with you . I have long wanted a yellow watchman goby . It is rare to see them here in our area , so one day I saw one I had to have it ! I knew my system could support him just fine . So I bought him ! I brought him home acclimated him and placed him into the tank .. lights went out for the night , so the other fish could go to there "area" of the tank he could find his "area" . The next morning I woke he was fine (used a flash light to see him) "sigh a big relief " go get my coffee , come back to the LR
the lights are on at 11 am in our system . By 1pm something wasnt right , so I start priming salt H20 prep for a change , get everything ready run some quick tests every thing is WNL and fine , but he is not I grab the hospital tank and some PVC
pieces and move to set up . I drain out 3 gallons water by this point he is just not right at all ! I pick him up and gently move him to his new tank . By the next day his back was red! What had I done ?!?!
For 2 days he held on .He was doing better on day 2 then BAM he was dead . I was perplexed ! Then it hit me I put him in the tank till his death ! I roomed him with a purple pseudochromis (mean lil bugers), he had invaded the pseudochromis's territory , now understand this tank is full reef so moving stuff was not going to happen ! And since it is a nano , "properties" are prime . The psuedochromis , and yellow watchman goby are same shape , size ect that could cause sparing ! I had at this point been doing this for 6+ years or so so I should have known better ! Moral of it is ....It happens even to us who have been doing it a while , and become lax .
Now to answer your question they usually do better in 3 or more but they may do fine in just the group of 2 .. That is upto you if you wish to add another chromis at this point .. GL HTH