You can swap the cogs on those? How do they come apart?

You can only maintain the benefits of HG as long as you maintain the intended relationships between cogs, so if you had a 28-25-22-19-17-15-13 and a 32-28-24-20-17-14-12 and you wanted to make a 32-28-24-20-17-15-13, shifting would suck between 20-17 or 17-15, depending which 17 you used. Merging HG cassettes only works when there's three consecutive cogs of the same size between them.

IMO you should quit futzing around with freewheels and rebuild the rear with a freehub; there are bound to be more options that way. Prolly get away with the same spokes.