With the "6 small Destroyers" you probably mean some ships comparable to the
Type 23 Torpedo boats. Well, if you envisage them to be laid down already in 1920 then they will look even more somewhat like
these. About their weaponry ... the 5'' guns were even in 1925/6, when the first bored out version of the 4,1'' gun were tested by the Reichsmarine some completly new and to the germans rather unknown calibre, when designed/thought of IOTL a reasction to the Washington treaty.
Much more sense would make it, if these new /destroyers" might be build with 4,1' guns but planned for to be equipped with
these 5,9'' guns which weighted almost the same as the
4,1 guns of the T 23 boats of OTL, somewhat lesser range but quite heavier a punch. If 'upgrade' to the weight of
OTL german 5'' installation with perhaps a wee bit higher elevation mount and/or a couple of more calibre lenght the range can even be improved.
Though I render your assumption of Scheer and consorts thinking of double purpose guns already in 1920 (
IOTL they were first conceived by the japanese in 1926 only as 5" guns, prior to midth to 2nd half of the 20ies heavy AA wasn't really a too important topic) ... somewhat bold, these 5,9'' guns would have made a great base for a double purpose gun. IOTL the german 5,9'' AA guns developed as such were not truly worth the effort compared to the german 5'' AA guns with its almost identical performance regarding reachable ceiling. ... but here we are looking for a true and full power
Double Purpose gun and as such the 5,9'' would have the punch of such a gun against other ships AND an AA performance similar to the IOTL best german heavy AA gun of 5''.
IMHO worth a thought ... especially if your development cycle for a proper mounting starts as early as ITTL.
Your assumed 'cheating' with weights ... why not also excluding not only reserve boiler feed water (as of the later Washington Treaty) but all boiler feed water ? ... if we are already at cheating (
the common german practice still executed IOTL with the Emden calculated 'construction weight' which includen all boiler feed water as well as about 1/2 to 2/3 of ammo and fuel).
Also have a wee problem with your freeboard 5 feet higher then what to expect. Man that's almost a complete additional deck ! ... and we don't wanna have the NIACC commissioners have to drink lead paint, do we ?
With this a proper placing of the belt armor is practically impossible. it is either waaay down the waterline looking false for an external observer or waaay above the waterline looking as false. However IMO two ways to go around it :
- don't make a belt-protection but 'simply' an internal box protection around the magazines
and/or
- plan an armor scheme right as with the former 'protected cruisers' the left small cruisers were built with.
You still plan to upgrade them later intheir history and it might let look you 'better' with the NIACC in that you trule 'only' replace the ancient small cruisers
.