If i use the others TX and RX as INPUT/OUTPUT, will i have problems with the Serial comunication to the PC's screen?
No, you won't. The RX and TX pins have resistors included in the circuitry that can present challenges when using them as input pins. I don't recall, and am too lazy to search, whether the RX1, RX2, RX3, TX1, TX2 and/or TX3 pins have the resistors, too.