I have never liked the ISC Tugs that have been around for SFC: CE/OP and I had ideas for a captured Merchantman Tug for a long time and here's a peak at what I have been up to...
In my corner of the Trek Multiverse, the ISC planners of the War of Pacification looked at what ships and resources would be easiest to convert to their needs in the territories they set out to pacify. The ISC would have had enough planning time to know in advance what Kliingon and Federation tug pods and even what type of mobile bases would be easiest to convert to their needs upon them being captured. As the Merchantman freighter is one of the most common non-military ships available in the Alpha and Beta Quadrants, it makes sense that the ISC could have acquired many of these vessels and a variety of Federation and Klingon tug pods prior to the WoP and started the conversion process ahead of the initial campaigns. These would be the basis for the Heavy Tugs and this allows me to use the earlier ISC Merchantman retextures as quickly captured ship prizes for the IFAS or even light Tugs.
What you can't see is the bottom of the Klingon Tug pods has been converted to a thru deck hangar. I have always envisioned that the ISC had millions of drone fighters at their disposal for their WoP and that whenever possible their ships try to capture their enemies ships and resources to augment their own military strength.