if your Nitrites are at 0.5 PPM you will probably have to do several large water changes. It sounds like your bb colonies are becoming established. You're probably 1-2 weeks away from being fully cycled. When you're fully cycled your Ammonia \ NitrIte levels will be 0, and your NitrAtes will be where ever you keep them at.
In a 90 gal you can expect about 5 - 10 days for the tank to adjust to a new fish, I wouldn't add more than 3 at a time, unless they are schooling and need 6 min.
Keep an eye on your barbs too, swimming in 0.5 ppm of Nitrites will poison them. Their gills will turn from a rosy pink to a brown or yellow color, if that happens, you can remove them. Your tank is probably far enough along to finish fishless, or you can salt your tank at 0.3 oz / gal, this will help neutralize the compounds that prevent your fish from absorbing O2, it'll also encourage their slime coat to grow thicker so they are less likely to be attacked by bacteria \ parasites. If you salt the tank, be sure to add plenty of airstones, as it will be heavier and less saturated with oxygen.