The idea is that The Fortress is a force on the edge of the Imperium. (Sticking to GW canon is not very important to me...) They survive and remain strong by hiding, raiding, being strewn over many worlds, etc. It is the only way to defend their worlds.
I'm going with a very simple tan paint scheme. The main figures are the Fortress Figures hovertanks. I'll be using the more recent version of GW Imperial Guard. The old ones will be IG, the new ones will be Fortress soldiers. I've got some micromachine Jawas which will be psykers. Battletech hovercraft (Savannah Masters) will be really small craft. I'm sure I'll use some mechs for titans, too.
The paint scheme so far has been very simple. Prime, spray tank, gray hover skirt, brown wash, bit here and there. That's it.
Pictures so far:
Here is a conversion of their missile tank using battleship turrets and card.