If I'm to avoid tumbling them together, I can afford to get only a medium capacity tumbler...
You shouldn't ever tumble them together no matter the size of yuor tumbler or your wallet.
Any time you attempt to tumble brass that has different mouth sizes -- where one can fit inside another, you've got to separate them and tumble each on their own. Quite simply, the smaller ones find their way in to the larger ones and the smaller ones don't get clean.
Not a concern with bottle neck rifle brass, where even a .22 Hornet is unlikely to find it's way inside a .300 Win Mag, but with handgun brass, the silly things are like magnets inside each other. If you tumbled 50 pcs of 9mm with 50 pcs of .40 S&W and 50 pcs of .45 Auto, you'd be shocked at how many of them are all nested inside each other at the end of a tumbling cycle.