First I'd like clarification on exactly what items you have. You say 4 x SP120 RGB fans, but Corsair has two of similar model names. The SP120 RGB LED High Performance fans are 3-pin fans (Voltage control Mode, not PWM) with plain RGB lights in the frame. On that lighting system, at any one time all the lights in a fan are the same colour, although over time that colour can change. The iCue SP120 RGB Pro Performance fans also are 3-pin (Voltage, not PWM Mode) and have the newer Addressable RGB lights. In that system, at any one time there can be several colours displayed in ONE fan. So, which do you have? And, in adding fans, be sure to get the same type.
For both of those fan types, the max current spec for the motor is 0.3 A. (Power for the lights is done separately with a different cable connected to the RGB Hub.) The Commander Pro specs say that it has six fan motor ports with standard 4-pin fan connectors in them, and each port can supply up to 1.0 A max. The limit for the entire Commander Pro is 4.5 A max for fans. So you COULD use standard simple 4-pin fan Splitters (NOT a Fan Hub) to connect two fans each to three of the ports, and the other three to three more ports, giving you 9 fans. That would present max loads of 0.6 A on three ports, 0.3 A on three, and total 2.7 A.
Now to the RGB lights. You have currently four fans plus four RGB light strips connected to a Corsair RGB Fan LED Hub, correct? Although the Hub has only six output ports, you mention the light strips are "in-series"; I presume that means some are plugged into others, in a daisy-chain arrangement. Maybe two sets of two. Anyway, the spec for that Hub is "six strips", but you seem to have eight lighting devices on it. Certainly you cannot add more to that one. You WOULD need to buy a second RGB Fan Led Hub. The Commander Pro unit has TWO RGB lighting output ports and is supposed to include two cables to connect each of those to a RGB Fan LED Hub, so you could arrange two RGB hub units controlled by the RGB ports of the Commander Pro. Then you could re-arrange the LIGHTING connections for 9 fans plus four strips to fit on a total of 12 outputs. The unanswered question then will be co-ordination of the light displays. The two Hubs each will be fed by a separate RGB channel from the Commander Pro under the control of the iCue software. I do not know the details of how that works. It may be that each channel is controlled separately and you cannot make ALL of your lights do the same thing. In fact, maybe that is what you would want. But there MAY be a way in iCue to sync both RGB channels to do exactly he same thing.
Now, on to your Question 2. The answer depends on exactly which type of fan you do have, and in understanding what each can do. In RGB lighting units there are three colours of LED - Red, Green, Blue. In the plain RGB system (the SP120 RGB LED High Performance models) all of the LED's of one colour (say, Red) are connected together and do exactly the same thing. Similarly for Greens and Blues. So along a light strip (or around a frame of a fan) the entire strip will be the same colour at any moment. That colour can be changed to a lot of colours over a time frame. In the more complex Addressable RGB system (in iCue SP120 RGB Pro Performance fans) the same three LED colours are organized differently. All the LED's in the strip are in Nodes, each containing one LED of each of those 3 colours, plus a controller chip for that trio only. Along the strip all the controller chips listen to the Control Line. The RGB controller box sends out on that line data packets of intructions, and each packet has an address for which controller chip should do what the instructions say. Each controller chip responds only to a data packet with its address, hence the name Addressable RGB. So along the line, at any one moment each Node of three LED's can have a different colour from all the others along the line. This can produce rainbow displays, and rainbow bands chasing each other down the line, etc. Exactly what displays can be shown depends on how the controller box is programmed. Most such system do NOT give you tools to manipulate each specific LED, but you get to choose among many pre-programmed patterns.