No need to adjust humidity unless it gets too high. With indoor the only humidity should be caused by heat, soil, and plants. Humidity is low right now because the clones are small and the space is large in comparison. There is nothing wrong with low humidity.
If the tent is sucking in, it means there is negative pressure. That isn't a bad thing either, unless you have a co2 tank and are recirculating air. If you have fresh air going into the tent, don't worry about co2, or losing co2. I prefer negative pressure personally, as I know whatever is being exhausted, if going through a scrubber, will have zero smell, and the smells in the tent aren't escaping into the house. This allows guests, repairmen, whoever to come over and have no idea what you're doing.
If your inline fan is 6", use 6" for both intake and exhuast. You want an equal flow for both for great air exchange. Maximum air exchange has tons of benefits, from giving more co2, to battling powdery mildew, to controlling temperature, to even getting good results with hotter temps (summer).
