As far as I know, normally when you directly supply the circuit, it must have decoupling capacitors to deal with noise. If I use an LDO instead of directly supplying it, do I need to have decoupling capacitors? In most datasheets they will recommend the best value of capacitor for best stability. Since inside most of LDOs is a CMOS FET, I am concerned that adding more capacitance (decoupling) than recommend is going to mess up the capacitance of the LDO input. Am I just worrying too much?
Here is my circuit LDO is driving ICs about 350mA max, supply is given to the terminal block (J4):
I am concerned that adding more capacitance (decoupling) than recommend is going to mess up the capacitance of the LDO input.
You can't have too much capacitance on the LDO input. This capacitance just provides a reservoir of charge for the LDO to draw from when it needs to supply current to its load. A bigger reservoir won't cause any problems with the LDO.
What it might cause is a high inrush current when you first connect or turn on the external supply. Whether that's a problem and how you might want to deal with it depend on what's providing the external supply, how much inductance is in the wires between the supply and your board, etc.
If I use an LDO instead of directly supplying it, do I need to have decoupling capacitors?
Yes, because LDOs generally aren't designed to regulate away high frequency noise. This makes sense: high frequencies can be attenuated easily by adding capacitors, and that is a lot simpler and more cost effective than making a faster LDO.
As @The Photon points out, LDOs don't mind large capacitors in the input, but the rest of your circuit might.