Recording from external synths, loop tips

To avoid latency, use the External Hardware Instrument device. Drag it onto a midi track.
Generally, record the audio to a seperate audio-track, then disable the midi once you're done.
Preferably record pre-fx, so you get a nice dry-signal.

A loop note/trick

-When the loop first starts, any reverb/echo will not be present. This can be weird, so it's worth recording an extra measure to use . Eg if your loop is 4 bars. Record 5 bars. Ableton can be configured to start at bar 1, but use bars 2-5 for looping. This way you get both a natural start and a natural loop.