According to Spinward Marches Campaign, 212th is identified at Rhylanor at the outbreak of the FFW (1107), with the 23rd at what appears to be Frenzie in Vilis subsector. My beat up ol' Supplement 9, Fighting Ships, confirms 212th at Rhylanor: "At present, only one Tigress class BatRon is deployed in the Spinward Marches, assigned to 212th Fleet, at Rhylanor. Additional Tigress class BatRons are generally assigned one per sector. Deployment: In war, the ideal deployment of any BatRon is together, as a unit. In peace, various Tigresses are often scattered throughout a region on peacekeeping missions, or to show the flag. Several individual Tigresses have been deployed among the worlds of the Five Sisters subsector to enforce the amber zone blockade of Candory and Andor."
(The use of past tense may indicate that they were deployed there in the past but are not deployed there as of the date of Supplement 9.)
According to MegaTrav's Rebellion Sourcebook, 23rd is at Rhylanor as of 1116, prior to the onset of the rebellion. I believe there was some sort of administrative restructuring after the FFW. I'm curious as to where you found a Supplement 9 mentioning the 23rd. Was there some late reprint of Supplement 9 with different info?
If you really want Tigresses at Candory and Andor, they could be on permanent independent assignment, not attached to any squadron but answering directly to 208th fleet HQ at - Karin? - and therefore don't count toward that 1-per-sector thing. Those two red zones are rather special; the Tigresses with their large number of fighters would be particularly well-equipped to patrol it. However, a decent carrier could do the same thing. I don't see opposition in that quarantine that would require a dreadnought's armor and firepower.
You could also decide that they come out of the Trojan Reaches fleet. I don't know much about that sector but it doesn't look like it's facing any major opponents, so it may have capacity to detach a couple of ships from a squadron to help the Marches fleet out with a quarantine. Unless something's going on in Urnian subsector over in - Foreven? - Five Sisters doesn't look like it would need a significant force other than for the two red-zone blockades. Running a couple of dreadnoughts on detached duty from Trojan Reaches in support of 208th would allow the Imperials to run an understrength fleet in Five Sisters: a couple dispersed cruiser or carrier squadrons - dispersed squadrons of Arakoine-class strike cruisers perhaps (seems a fair use for an unarmored cruiser whose only other role would be to threaten inhabited worlds that lacked much ability to shoot back), or Wind-class strike carriers - with individual ships posted among the various Five Sisters systems neighboring Candory and Andor to watch for ships that might try to jump to or from the two worlds, along with the support ships and administrative structures needed to support the red-zone.
