It always seemed to me that the reason space hulks exist is for the same reasons oil and water don't mix.
Entropy increases when the two substances cease to be mixed, I.E. Immatterium becomes more disordered when the ships amalgamate together.
Presumably this means that so long as there was enough energy being input into the system (the ships towing through the warp) then you would be fine, but this would mean a shit-ton more power than normal. I would think that these tow-ships have no weapons and instead sacrifice better engines, weapons systems, and possibly a lot of secondary systems to maintain significantly larger warp drives and Gellar fields to be able to do this.
From this it means that it is not so absurd that ships get towed, but likely the jumps would have to be short, with long wait times, probably only 5ly at a time, rather than the hundreds ships can perform normally.