that is a tricky problem. at my old job we had a similar problem.
with most 4.1.X releases we had HA problems, and ended up running all our cores on a single SF. it was more stable than running with two SF in HA mode.
as for your mis matched SF's, I don't see a problem. as long as they are running the same code. in that case I would would run 4182.
another options.
if the two core 8610s are in a full ist/smlt redundant environment I would remove the 8691's and divide up your 8692's for your two 8610's. upgrade them to the latest v5. then talk to finance and order two more 8692's, you can just add the stand by SF after the fact.
