You should get more red's but first you need a bigger tank! I would suggest 25 gallons per fish, most say 20 or even less per fish but if you want them to look natural and actually beable to swim around. It is best to go as big as you can afford. So I would say buy two more red's and get a 75 gallon tank at the very least.
good luck... Jake
Yeah 3 RBP would be very nice to have... however keeping 3 of them in a 10G tank will prove to be quite a challenge. If you keep them in that tank together you will need to do what seems like endless WC to keep them alive and also they will more than likly start canibolism. I personally would stick with just the one RBP now until you can grab another tank
10 Gal is good enough for your 1 RB for now. Suggestions for more than 1 would require for you to get a bigger tank for more space and territorial disputes. If you just want 1 aggressive fish, stick with the Rhom. Suggestions to get a 7"inch or more. They grow rather slow.
The fact that your RBP is only 2 inches after 5 months is evidence enough that it needs a bigger tank and some friends to move around in. With proper space and nutrition, you rbp couldve been around 5 inches by that time. Get a 55 g and put 3 rbp in it.
5 2" ers in a 10 gallon? If you pulled that off I bow to you because IMO that is impossible and I feel bad for your fish. Don't get more fish until you have a bigger tank.!!!! If you want a solitary fish that isn't a p*ssy and can stay in your 10 until you save money for a bigger tank. Go with a rhom they stay small for hella and then you can get him his 55 by the time he needs it.