1- From every BL book with a techmarine character in it. They usually have more than "basic" engineering skills as well. Plus, SM, especially when they have a good deal of resources at their command, often refit things to suit their own needs. Look how many "unsanctioned" variants of the Land Raider there are. And given time and a need, which SM always have both of, refitting star ships isn't that far fetched. Prime example- The Silver Skulls Strike Cruiser Dread Argent, which I now kinda want to add.
2- Since you can already take each class of those ships and/or any neccessary refits to make them, and they are unnofficial anyways, I see no reason NOT to do attached commanders. They are fluffy and fun and allow you to play in new ways, not limit what you can do.
3- I added the name Ragnarok because plain "Assault Battle Barge" just sounded lame. It is for use with any and all SM fleets. The fluff will likely just be that it was first developed by the SW. It is kinda like the SO. What was originally just ultramarines is now widespread.
4- As for making it generic, that is not the Point of ASC. The point is to present as many options to the player as possible to make their games as diverse as they want, and since none of the classes of vessels are SW specific (only the character ships), there is no reason that they can't be included IMO