PCB needs oscillator source too.
Well, set it up with the ICSP pins and the minimal circuitry. In summary:ICSP Pins as noted in originating post.0.1uF caps on power lines, 0.1uF cap of Aref, 10K pullup resistor on reset, 16 MHz xtal, 22 pf caps. If you plan to downloadl via serial interface at some time, add Serial Rx & Tx lines, and 0.1uF cap to a DTR pin, then can connect USB to Serial adapter via jumper pins: +5,GND, Rx, Tx, DTR.After that, whatever you plan to use:4-pin I2C connectors? +5, GND, SCL, SDA - Maybe add 3.3V regulator also and bring out 3.3V5-pin SPI connectors? +5, GND, MISO, MOSI, SCK, unique CS per connector4-pin serial connectors? Rx, Tx, Gnd (Serial1, Serial2, Serial3)Shift register connectors? Data-in, data-out, clock, latch, clock, +5
"Couldn't I just use the TX, RX pins and remove the UNO to upload code to my MEGA2560 on PCB?" I believe so, if your MEGA2560 has a bootloader already installed.
In my opinion, you are indeed thinking correctly on that.Do you have a 100nF cap from an external DTR pin to the reset line also? Need for automated reset durung serial downloads. Very handy.
Yes, I guess it would. But plan ahead leave pads for one.