If it helps, here's my experience with two ISY/Elk combos in the past, in previous homes. The ISY connects to the Elk via IP, and has no effect on the Elk's M1XEP connecting to Alarm Relay for direct monitoring. It just works.
That said, I can't test, etc., right now as we sold the last property with an Elk. Am thinking of replacing current alarm system (a Napco Gemini) with one, but have hesitated because I can't tell that Elk is as strong as they once were, as a company. If Elk had a decent dealer network (they sent me to a dealer who hadn't even heard of them) OR still had great DIY support like back in the days of Spanky it would be a no-brainer. But they seem to have an even smaller presence as a company than ten years ago, and a retail (DIY) network which has me holding back (dead links on featured dealer sites, sites from ages ago, etc.). It does not inspire confidence and has me worried about the manage,ent, to be frank. I sure miss the days when Spanky was active, I can tell you that. I'd have had an Elk installed the first week in the new property if that level of support and customer interest still existed.