Custom slots are good. Especially if you're using a component that each race has different versions but you want to limit their number in a ship. If you just put "Only 1 component allowed per ship" restriction you can still have 1 from every race. So you need a custom slot to do that. I made one for TPM sometime ago.
http://koti.mbnet.fi/sfhaven/aarchiv...pucomptype.jpg