Re: 0.5mm rashguard and ...
Wetsuits work by trapping water against your skin and letting your body warm it up, and blocking the wind.
Fleece lining works by trapping air against your body (and blocking the wind).
I think a fleece top would probably just become waterlogged. To the extent the water is kept against your body and warms up, then it will help, but if you are wearing it over the top of your rashie then I'm not sure how much it will assist. Might work a little bit but will add some weight.
Compression shirts are usually designed to wick away moisture, so would work the opposite of the wetsuit. Not sure how this will go, never tried it.
Why not just get a 1mm or 2mm top? No point in saving a small amount of money for a solution that constantly annoys you