First of all, just a little clarification on nomenclature. Feral means
living in the wild but descended from domesticated individuals. So once you but a swarm into a hive it is no longer "feral", and as Perry inferred, once a swarm leaves a hive it is "feral".
I draw a distinction between "feral" bees and what I refer to as "survivor" bees. I consider survivor bees to be those that have been feral for a year or more (no treatment from beekeepers). The only true way to get survivor bees is from doing a cut-out that you know has been there more than one year. Or of course if you witness such a nest swarming. Otherwise, I consider all swarms to be non-survivor (chances are they came from another beekeeper or pollinator) Pollinators are notorious for letting swarms go. Farmers want their moneys worth and want to see packed hives. Once these hives hit the crop they boom and quickly become overcrowded and swarm. Pollinators don't have the time to chase swarms. I know a guy in Georgia that chases swarms in canola fields and gets upwards to 400 swarms a year (then sells the hives back to pollinators
)
I am a strong believer in survivor bees and have been keeping them for close to 10 years or so. Long before it was popular to keep "mutts". My experience has not lead me to believe cell sizes play a significant part. When I first went the route of migrating my stock to survivor stock, I focused on doing cut-outs of those year or older feral colonies. I would put these cut outs onto Honey Super Cell as I "thought" 4.9 was part of the solution. It got towards the end of the season and HSC was out of stock and not shipping, so I had no choice but to put the last couple of cut-outs on fully drawn 5.1 comb. These hives came through winter and performed equally well as the HSC hives, and from that point forward I have not purchased and more HSC and use 5.1 foundation and still remain treatment-free.
A couple of other points mentioned. I also agree with Perry on keep acclimatized bees. But collecting local survivor bees you get that by default. Long before 9/11 caused David Eyre to stop shipping queens to the states, I had much better success with his northern queens than with any I could get from the south or California. In fact it was the inability to get queens from David that drove me to dive into the survivor realm and start raising my own queens from survivor stock.
I am also a firm believe in the power of the nuc. I can't explain why and can only speculate, but it seems that an overwintered nuc will grow fast in the spring and end up out performing a full size hive that season. Nucs bring tremendous advantages not only in spring build up, but also brood breaks to help with varroa. I believe these brood breaks are key to the feral bees defense against varroa. Feral hives swarm at least once a year and get this brood break. We as beekeepers try to prevent our hives from swarming and never get a break in brood.
That's my story...... your mileage may differ