Could i do the conclusion that if it has a micro SD slot then its compatible with Mega?
I'm not sure if this coincidence is a correct assumption. If you have the schematics, then look if the SPI lines are from the ICSP header and not from the corresponding digital pins (which are 11, 12 and 13 on the UNO but 51, 52, 53 on the Mega).
So if i don't have UNO rev3 i wouldn't benefit from latest rev (6)..?
The Mega2560 in the newest rev (3) uses the same pin layout. As fas as I know rev 5 work withe the Mega series too, but I don't have detailed information on the revs before that.