MyRoleplay stopped reading other flags after patch 4.0. The optimal thing to do now, if you want to see everyone, is indeed to run both. I have been running MyRP along with FlagRSP Cataclysm (available only on Wowinterface, not on curse) without the game imploding on itself. The two work together. The only problem is that -sometimes- the tooltip will go blank and revert to a default one, but the descriptions and icons will still appear on the target frame.

So far as I know yes, Flag will read other RP addons like MRP, Immersion, Mary Sue, etc. We were testing this out after people were finding working RP addons post-4.0.

@Raventis - the addon that I really miss after installing MRP is Vlud's Target Sex Mod. VTSM shows the gender of the target player, and is most useful for the holiday quest for Noblegarden where you have to put bunny ears on a female of each race. However, from a roleplay perspective it also prevents you from rolling up to that BElf or orc and saying "Old woman!" when in reality it's a 37-year-old man. Named Dennis.

Thanks for answering!
I really like FlagRSP2, but I also run FlagRSP. And I think I got the Cataclysm version via curse client -- it updated and now says Cataclysm in the addon activation menu. It has the tooltip bug you mentioned, but also sometimes fails to show descriptions (like in: I see the description, but next time I click on the player, it's gone. Really annoying).

Yep, that disappearance issue is the result of having both addons running. It's a choice of either having that disappearance issue, or only ever seeing half the people. It's really frustrating.