Looking more closely, your question appears to be "does the ADS1299 contain everything I need, or do I need more signal conditioning in front of it". The ADS1299 has the features discussed in general terms in this paper: http://www.ti.com/solution/ecg_electrocardiogram. A paragraph salient to your question appears under heading "Signal Acquisition challenges", which I'll expand on here:
"Some of the 50Hz/60Hz common mode interference can be cancelled with a high-input-impedance instrumentation amplifier (INA), which removes the AC line noise common to both inputs."
This deals with "Common mode rejection ratio" of the amplifiers themselves, used in conjunction with one another. The assumption is that a noise signal coming from an external source (say 50Hz power line noise coupled to the patient body through the capacitance between body and nearby power wires) will create an equal signal at each electrode, which should subtract out, since the amps are used to measure voltage between electrodes. However, differences in impedance or amplification between the amps will mean that the not-exactly-equal large noise signal at two different amps will subtract to a difference that is significant compared to the small desired ECG/EMG signal. "Instrumentation amplifiers" are designed with good matching, so as to have good ability to subtract (reject) this large "common mode" signal. Point one in favor of ADS1299.
"To further reject line power noise, the signal is inverted and driven back into the patient through the right leg by an amplifier."
To reduce the actual common noise signal that all the electrodes see, the ADS1299 includes the ability to detect the common noise signal, and apply the inversion of that signal to the patient's body ("right leg drive, RLD": http://www.ti.com/lit/an/sbaa188/sbaa188.pdf). If this strategy were completely successful, the power line noise would reduce to zero, and there would be no noise to worry about. Evidently this is not feasible, but at least it greatly improves the situation.
"Only a few micro amps or less are required to achieve significant CMR improvement and stay within the UL544 limit." ... and the feeding of the RLD noise-cancelling signal to the patient doesn't violate safety rules.
"In addition, 50/60Hz digital notch filters are used to reduce this interference further."
Once the data is digitized you can always apply digital (software or DSP) filtering. This is not part of the ADS1299 per se, however features of the ADS1299 help make this feasible, as follows. In some systems, digital filtering is not an option if the 50Hz signal is large, because that would require the gain of the amps to be turned down so that the large input signal is still within the range of the A/D convertor. This in turn would mean that the desired ECG/EMG signal would be proportionately smaller, perhaps so small as to be unusable once digitized. A previous paragraph mentions the benefits of 24-bit A/D conversion relative to 16 bit, and this is a place where it comes into play. The additional 8 bits allow good resolution of small signals even when they accompany some large signal, giving you the opportunity to filter out the unwanted large signal in software.
My conclusion is that this device is intended for use without additional front-end conditioning (ie: 50Hz noise filters). Of course this should be considered in light of the signals you expect -- voltages of your signal relative to those from ECG or EEG. Also, you'll want to assess whether the sampling rate is suited to the pulse shapes you see in EMG (as opposed to the more continuous lower-frequency signals of ECG and EEG).
Hope that helps.