well you will have no problems at all with bioload. That will not be an issue at all. The problem will just be the amount of fish and the space available, you know? I dont know much about purple psuedo's but most other psuedos that i've seen normally require a minimum of 30 gallons (for the smaller ones) but again, i'm not sure about the purple. But if it is anything like the others, then no i would not add anything.
But imo, you may be able to get a firefish or maybe a smaller cardinal. The all stay more up in the water column and dont bother other fish. Most other fish stay closer to the rock, so its not like they would really clash much if at all