I can't comment on the effectiveness running Abaddon and Mortarion in the same army, since I use neither of them, but my Death Guard army is based off the Chaos halves of the Dark Imperium and the Know No Fear starter sets. Combined they make an OK force. There are a few things I found that made it a bit tricky to work with though.
The main letdown for me is how terrible the Lord of Contagion performs on the tabletop. He is a point sink, but if you replace him with a cheaper (or better) HQ then you'll be better off.
As fun and fluffy as 7 Plague Marines is, it's a really weird number to work with. The sensible thing to do would be to get some spare plasma guns and have 3 of them (2 on the Plague Marines, one on the Champion). I put a Plasma Gun and some spare bits on a regular Plague Marine and now he's my Champion. Because I had the Know no Fear set I ended up with 12, but usually run a single squad of 10. It's pretty expensive but I find it works pretty well.
I'm also not a fan of the Fist+Sword Plague Marine Champion. Cool model, horrible combo. You could pretty easily put a plasma gun in his sword hand if you've got any spares, but I chose to use the Champion a counts-as Chaos Lord of Nurgle to replace the Lord of Contagion and give my units of Plague Marines re-rolls.
Given that you need 3 Troops choices to field a Battallion you either need to break down the Plague Marines into squads of 5 (buying the box with 3 marines in it is a good way to do this) or have 2 smaller units of Poxwalkers. I find Poxwalkers work best in big squads. 10 man units die very quickly.
From there I bought more poxwalkers (I had 30 from the two starter sets, I got more from the Easy-to-Build box and some old models I had lying around so I can use 2 squads of 20), more Bloat Drones (I have 5... I bought them before the "Rule of 3". Do not buy 5....), some Myphitic Blighthaulers, and re purposed my old Helbrutes to Death Guard.
I sink a LOT of points into the Helbrutes and Blighthaulers, which aren't super good, so you could probably fit Mortarion in pretty easily instead.
Abaddon gets tricky, since you'll need another detachment for him and his rules work best when paired with Black Legion. If you can get your hands on some Cultists an effective (and imo boring) way to use him is among a few units of them - but if you're really keen on using him without any Legionnaires you can just fit him in as a single model in an Auxillary detachment.
Edited by Fhanados, 10 May 2019 - 05:30 AM.