You mention sanding on a downdraft table. Are you wanting to connect to the downdraft table or to the sander?

If connecting to the downdraft table, use a standard dust collector. More CFM is better and if this is a home-made downdraft, you can add more holes or make it larger so your collector pulls more CFM (more holes = less restriction = higher CFM from the fan).

If connecting to the sander, you want a shop-vac style because you will be using a 1-1/4" hose. I would not want to wrestle a sander with 2-1/2" hose.

I didn't know the Oneida collector filters are HEPA rated, maybe so now? The Festool vacs have HEPA 2nd stage filters. Although the vacs are expensive, they are very quiet and replacement HEPA filters in those have to be much less than a HEPA for a 3HP collector. In general, sanding dust is too fine for consumer grade cyclones anyway, a lot of it will end up in your filters, blinding them. Also, when you pull less CFM, the cyclone efficiency in drastically reduced which will allow even more carryover to the filters.

Mike