Time To Add More Fish?

I'm currently now almost 3 weeks into a fish in cycle with 10 Cardinal tetras in the tank. I followed the advice from the shop and started the tank with API  quick start. I added the fish after 4 days, also as instructed by the shop and added more quick start as per the instructions.

I've been testing every day and doing 15% water changes every day or 30% if I missed a day. The ammonia has never been higher than 0.5ppm and the nitrite seemed to peak at 1ppm the middle of last week.

Now on Monday evening I did a 30% change and took readings at Tuesday lunchtime, Ammonia  less than 0.25ppm, Nitrite 0ppm, Nitrate 10 ppm. I did the same again in the evening with the same results and they've stayed the same all week. My local water has an average Nitrate reading of 7ppm and my water is soft. The tank is a 90l Fluval Roma with the U2 filter.

So my question is would I be ok to add some more fish this weekend and if so how many? I'd really like 5 banded barbs but they liked to be kept in larger numbers. A bit stuck as what to do. Any advice appreciated.

Thanks, that's just what I was hoping to hear 

I suppose the barbs will be ok in smaller numbers until they get a few mates,  happier than they would be in toxic water anyway.