Well, it's been quite a while, but I made some more progress with these armies over the last weeks.
@Dr. Zombie: thanks for the ideas. A waagh banner seems to be a must have
Assault Marines with an extra base including a chaplain
The chaplain was made using a plastic chaplain and assault marine, chopping both up and gluing the right parts back together.
Scouts, made from regular marinessince I couldn't find any (for a good price) on ebay. I cut the missile launcher back to a box and drilled a hole in it (had seen this before by someone else), cut of the clips of the boltguns to give them a more slender profile befitting a combat shotgun and gave them a different paintjob to fit with their tactical role: german grey, nuln oil, again german grey and a light drybrush with codex grey. I painted the shoulder pads the same way as my other marines to tie them in with the army.
I also built a Mega Gargant I got off ebay for 20 euros including shipping and it was still on the sprue
Quite an enjoyable build. And with it being the ride of my warlord, I made sure he would still be leading his boyz from up high and in person. It seemed more fitting for him then being hidden somewhere within the large metal hulk
Still need to do some work on it though, like the small turrets and drilling out the barrels.