It's my understanding you can't do a Custom Install from 32 bit to 64 bit. A clean install, by definition, means everything gets wiped. It doesn't mean 32 bit programs won't work. I have Vista 64 bit SP1 and nearly all my 32 bit stuff works. It's the 16 bit stuff that won't work without some emulator like DosBox or some other Virtual Machine. 64 bit Windows has 32 bit emulation built in(Windows On Windows.)
Most video stuff hasn't been updated to 64 bit anyway. People doing video conversions etc. on 64 bit systems, except for a few trail-blazers, are using 32 bit software.)
As for specific retail software I would go on the particular site and see if you must run a dedicated 64 bit version or if you can use the 32 bit. Unless something is running something pretty low level(like if it requires a driver
) it should work on the emulation.