hey , i have a question. or two
1. do i have to catch a pokemon and check its IVs every time i want to hatch a shiny or is once enough.
You'll have to check the IVs of a caught pokemon each time you want to lock into a shiny egg so that you can find your seed to figure out the number of taps needed.
If you are IV abusing then you should check your seed each time in the same way as stated above, but you could use the "coin flip trick" to find your seed, but you would still have to check your egg for IVs to make sure you still hit your frame. The coin flip trick will not indicate whether you hit your frame, only if you got the correct seed. Check the OP, it should be in there somewhere.
2. i keep running out of rare candies before metal kids IV checker can give an accurate result, i've been told to iv battle, so will that advance the seed and stuff etc
No, you cannot IV battle, that will randomize the seed (I think), but in any case an IV battle will not work when you want to lock into a shiny egg, and will force you to save if you are checking an eggs IVs meaning you are stuck on that particular egg.