The smaller the bit the more fragile it becomes so we would recommend keeping the via size above 0.6mm. 0.7mm or 0.8mm would be safer.
We have not shopped around for bits too much so we can't recommend one brand over the other, but for what it's worth the last bits we ordered we got off Amazon. Here's the link.
A quick search reveals that Digikey also has them, but they seem a tad expensive.
The insulating ink had many challenges, using the 0 ohm resistors is probably the easiest at this point.