The 51 cm Shallow Water works well with a 9.0 Retro.
The 44 cm might be slightly small for your big Retro, but if your water depth is < 55 cm you may not have a
A really good alternative that is as good or better but in a smaller package is the Lessacher Duo Formula weed at 40 cm. I use this fin all the time and the concave/convex "Duo" foil really works and the fin is quite fast (for a weed fin especially) and points very well for a weed fin.
Another US brand that works extremely well is the Tangent Dynamics Reaper.
I use a 46 cm for wide boards (> 86 cm) and for your Isonic 122 (I've sailed the 122 extensively in very shallow water at Lac Bay in Bonaire) the 39 cm is perfect for 7.5.
The Lessacher duo 36 is also really sweet on the iS 122/121.
Hope this helps,