As for playing along with audio from your mobile device (smartphone, tablet, etc.), you will not notice any latency in a bluetooth audio connection. In this scenario, it doesn't matter whether it were 5, 50, or 200 milliseconds of delay. Your perceived real time begins from the moment your speakers in your playback sound system emit the sound --- not when your smartphone initiates the bluetooth transmission of the signal. Any delay in the reception of the bluetooth signal by whatever bluetooth receiver you might choose (to use to connect to the Helix's FX Return 1/2 or FX Return 3/4) will be irrelevant.
The most commonly suggested method to connect a smartphone to the Helix is via the Helix's USB connection. This conveys Stereo digital audio to Helix via USB Channel 1&2. This method requires the Apple camera kit, which I understand is essentially a USB on the go adapter for Apple gear.
If you wish to use an Analog stereo headphone 1/8" jack as found on most all devices -- apple's latest excepted -- simply get a stereo 1/8" TRS to Dual 1/4" L & R TS cable. These are very inexpensive and work wonderfully with the Helix. Also convenient is that via this method you can adjust the playback feed audio on your smartphone. My understanding is that with the apple camera kit method you cannot adjust the audio feed level on the apple iphone. Rather you have to do so on the Helix via the Global I/O for USB channels 1&2. The 1/8" analog stereo method is how I connect my Android or apple iPods to the Helix for playback and this method works great. The headphone jack method also works great to connect your laptop, tablet, computer, CD player, or stereo system all of which have a headphone jack. The USB method works great with a Laptop or Desktop, as well as if you get a stand along audio interface.
Hope this helps.