You're right about Succor 2, it is a larger ship than Succor 1. I'm still very surprised that I can build that at the Naval shipyard tooled for Ruin-1. It's a ship 15ktons smaller, with almost no overlap in components (1 layer of armor vs ~10, no shields vs 9, no weapons, commercial vs military engines, etc etc). Should it really be buildable at that shipyard?
Refit/interbuild is determined by the ratio of
new component costs to the total build cost of the tooled-for class. You don't show the Succor 2, but the Ruin 1 dreadnought costs over 20k BP while the Succor 1 costs 1,300 BP, so I would guess the Succor 2 at 65k tons probably costs less than 2,000 BP. Since the Succor 2 costs around 10% what the Ruin 1 costs, it can be interbuilt pretty trivially despite the two ships being nothing alike.
This is a bit of a gap in the realism of the refit mechanics, but correcting it would probably require either (a) limiting refit/interbuild to not cross military/commercial designations, which is more than a bit artificial, or (b) reworking the mechanic entirely to use, e.g., a per-component-type replacement scheme, which would be really cool but also would take a lot of work for Steve for really no gain in terms of gameplay.