I have a vortech supercharger and need an fmu. Will any 8:1 fmu work? Even if it was meant for a honda?

they're all the same... the only difference is the hoses that come with some to connect to the fittings.

Like size and the amount of pressure they can hold?

A lot of them are the same physical piece, even across brands. They have the same sizes for the connections on the body (1/8npt), but they could have any ratio disk in them that is available for them, typically somewhere from 4 to 12 : 1