Yes, adding another one would be better than replacing. The value is generic, can be anything of about 10...100nF low ESR, made for DC bypassing, like MLCC type. Sometimes you will see many values in parallel, 10nF, 47nF and 100nF soldered together, because each value has a resonance at a different frequency. 1uF is too big, OK only if this value is the only one you have available, but not recommended.
If the RAM chips have +12V and -12V, too, (soem have only +5V), then add 0.1uF between -12V and GND, and +12V and GND, too, near each chip, like you did for the +5V and GND.
Same on the +5V of the Z80, and near the video RF modulator, if there it has antenna out.
If the electrolytic capacitors are the original ones, just replace them with the same capacitance and the same voltage, preferably Low ESR, as Vovk_Z told you, too. Don't waste time measuring the old electrolytics, they are very cheap to buy new ones, just replace them. Even if they seem reasonably good today, later they can leak or dry out.