Ok so first things first, "shinyness" is determined by IVs in gen 1 and 2 on the virtual console. If the Pokemon has the right IVs to be shiny (each Pokemon has a specific IV shiny "code" as it were) then it will be shiny. However there are some Pokemon that can never be shiny, for instance, only the letters I and V for Unown can have the IVs for a shiny Unown (coincidence... right?). I know that in gen 1 it isn't possible to get a shiny, however if it has the shiny "genes", and it is transferred to a gen 2 virtual console game then it will be shiny. The conditions are:
- Its Defense, Speed, and Special IVs are all 10
- Its Attack IV is 2, 3, 6, 7, 10, 11, 14, or 15
This data is taken from https://bulbapedia.bulbagarden.net/wiki/Individual_values#Shininess.
There is a glitch where you transfer the Red Gyarados from the lake of rage in a gen 2 vc to a gen 1 vc, teach Gyarados mimic and then make a ditto transform into the Gyarados once and then twice, this will copy the IVs turning it shiny. All that is left is to send it over to a gen 2 vc and it is shiny!
Now you put it in the day care with any other Pokemon and the eggs you receive have a 1/64 chance of having a shiny inside!
The IVs are passed on from the male of the pair if breeding without ditto (the species is determined by the female and one of the egg groups need to be the same for the Pokemon to produce an egg), or the IVs are always taken from ditto if breeding with one.
Any Pokemon sent from the vc to ORAS, XY, SM or USUM have their hidden abilities so you can go for a shiny HA.
Couple things that you probably already knew, legendaries can't breed so don't try, and the egg that you get from the day care already already has a 1/24 chance of being shiny, so save before you get the egg, then get it and hatch it, and if it is shiny, yay, if its not then tap the lower screen, restart the vc and receive the egg again, repeat until you hatch a shiny (I got a cleffa).
Now seriously the last thing: you CAN CLONE ANY POKEMON USING A GLITCH IN THE VC, meaning that you can just get one and then clone it over and over.
Hope this helps!