I'm definitely not a Dat expert as much like Empyreal my personal experience is extremely limited as I've only kept three Indo's and never at the same time so take my advice with a grain of salt. In the first two cases they where kept with larger fish some of which where CA/SA cichlids and in my experience where at least as aggressive as the CA/SA's, which surprised me since I thought they where a shy, retiring fish in general.
With that said I decided to treat them the same as CA/SA's and used decor to try and create territories and break up line of sight and give the Dat a place to hide. In my experience it worked much better in curbing the Dats aggression then the the cichlids. As an ambush predator the Indo claimed what he considered the best hide and then only really got defensive/aggressive when another fish approached it. The Indo's basically hung around their hide only venturing far for periodic patrols and feedings instead of constantly patrolling and chasing fish most of the time before the hide. So that approach is what worked well for me in curbing Indo aggression and I'll also offer up that all my Indo's have been extremely stable which I credit(perhaps mistakenly, maybe I just got lucky) to being comfortable in my set up.
As for keeping more than one Dat I'd try adding as many juveniles as possible and keeping them in odd numbers and thinning the herd as they grow. Hopefully with a fair amount of targets and an odd number of them will allow the aggression to be spread between them. Another plus is as you thin them out as they grow you can keep the best tempered, stable and patterned of the group. Again just my opinion and I hope a helpful one.