Yes there are plenty of kanto and mostly johto Pokemon you can find in emerald most of them can be hunted down in the safari zone.
no but if you catch all the hoenn pokemon you can catch jotho and kanto pokemon
Its not possible to catch Ho-oh in emerald to get one receive one from Pokemon colosseum.
You go to the safari zone after you finish the game then, if you see a new path go to the grass and you can catch Pokemon from the jhoto region
You can found it in the Safari Zone, but to catch it you will have to have passed the elite four and have a Pokemon with rock smash and then smash all the rocks in the Orres Pokemon regions.
You can't catch one in Pokemon Emerald.
Yes, You can catch Groudon in Pokemon Emerald.
Actually, it's impossible to catch Milotic in any Pokemon Game. But, it's possible to catch its pre-evolution, Feebas. To catch Feebas in Pokemon Emerald, there is a complicated way. But, I know that you can catch Feebas in Route 119. Hope I helped :)
No you cannot catch Entei in Pokemon Emerald.
You can't catch a Bellsprot in Pokemon Emerald. You trade it from Pokemon Leafgreen!
There are many Pokemon you catch that are similar in sapphire and emerald but there are many Pokemon emerald can only get and not sapphire.
Cressilia is a 4th generation Pokemon so you can't catch it in Pokemon Emerald.
You can't catch or obtain other trainers Pokemon in emerald.