you can combine the two but the system will only recognize each gpu to have 1.5GB VRAM putting the extra VRAM on the 3GB version a waste. if you already have the one with 1.5GB VRAM just pick another with the same amount of VRAM. if you have the 3GB version don't waste your money by getting the one with 1.5GB VRAM to do SLI. as for the bridge i think you only need one in 2 way SLI
I believe Nvidia cards need to have the same amount of VRAM to be in SLI together. AMD cards in CF don't care if they are different though. Regardless, it would be pretty pointless to put a 1.5gb with a 3gb even if you could.
it depends on the card configuration. for example you can SLI GTX580 3GB with GTX580 1.5GB or GTX560Ti 2GB with it's 1GB variation but you can't SLI GTX460 1GB with GTX460 768MB. in short the card can be SLI even if the amount of memory is different as long as base configuration of the card are the same.