All of the Regi's are on level 40 when you encounter them.
lvl 40
you dont
in southern island there was latias and latios you will meet latias in Pokemon sapphire and latios in Pokemon ruby
His house in mossdeep city.
Well that is quite a difficult task my friend. It is required that you have Pokemon Ruby version as well, as some Pokemon are not found in Sapphire. I have completed the pokedex in Sapphire and I would be happy to help you with any direct questions about where to find a specific Pokemon or how to obtain an evolution. Just post a message on my message board or on this question's discussion page. :)
on the second gyms island the cave that you meet Steven, the hiker at the beginning will give it to you
professor oak only appears in diamond but if you want to meet him try sapphire version
Probably Smeargle.
after you beat the Pokemon league chapion may and professor birch will congratulate you. professor birch will also check your pokedex.
From Mauville City, go south, you'll meet the Fisherman. Surf and keep to east. Then, you reach the New Mauville.
The only way would be cheating as you can't breed legendary pokemon and you can only meet Regigias at a high level.
Sootopolis City.