Am I iv breeding my pokemon right and if I am can I do it more efficiently?

I think I am breeding by shroomishes right so far I have a male and female shroomish with perfect speed and attack. I am about to venture into the defenses I am aimimg for at least the mid 20s for defense and attack but HP I need the perfect 31 to get the 13% toxic orb health gain and sp attack I don't care about.

I am playing emerald and my method is to breed 2 pokemon with good ivs together and choose out the best of the bunch with the same good ivs as there parents and some new good ivs that they randomly got and then breeding it in place of its mother/father. I check for evs using my cloned rare candies via the emerald glitch where I check them all out at about level 20. Is that a good method or are there better? Also since this is for the 4th/5th gen metagame I have to migrate my pokemon over at some point when do you think it would be go to migrate them?

Also finally nothing to do with iv breeding but if you migrate a breloom with in a 3rd gen game is there a chance that in the 4th gen game it will have poison heal? Because I heard that migrated Clefables sometimes get magic guard does that happen with breloom to or is Clefable a special case?