Most (but maybe not all) canti studs have M6 internal threads. You can use any high quality screw you want, or just put a couple of washers on the outside to use fractionally longer screws you have.

Cable guides?? Do you mean straddle cable and saddle:
https://www.amazon.com/dp/B001CK0F0W...=IHKA3LF9CB68C

or hangers?
https://www.amazon.com/Tektro-Front-...B4EMKJ3Q5JVK1E
https://www.amazon.com/Origin8-Seat-...7JV60V5DSYR2BM
https://www.amazon.com/Tektro-Hangar...B4EMKJ3Q5JVK1E