I like your ships a great deal, Helmut. I've been wanting to see ships which appear thrown together, because that fits the lore of Starsector. The smallest one looks a bit iffy, because the size kills the style, but the rest are excellent.
The way in which you would balance them for gameplay via the art, by giving them tons of weapons but no rear PD, is a great idea. It specializes them, which is important. Every ship should serve a specific purpose. These clearly would.
My only qualm with your design is the contrast in detail from the muck of shaggy metal to some of the engines, weapon mounts, hangars, and a few other bits. It makes the mentioned parts really stick out where they shouldn't. I say it's a qualm, but it's a rather large issue.
Those sprites have great potential. The style you are going for is something I've wanted to see in this game since I've started lurking on these forums months ago, but that contrast is a killer to me. #5 is the best large one, because the engines fit the style, and the only major issue with detail is, again, the weapon mounts. If you can make the rest have that balance as does #5, these would be among my favorite ship art ever. Seriously, you do a fantastic job capturing that style and feel. Maybe just blur up the weapon mounts, and change the very dark grey in them to a touch lighter shade.
Good work though