Is it possible to breed Authentic Sinistea/Polteageist

It is not really possible to tell if it is too powerful without engaging the battle. Once you started the battle, you can tell by their level.

The ability to catch higher level pokemon is tied to the number of gym badges you acquired so far. It goes as follows:

• No Badge: up to level 20
• First Badge: up to level 25
• Second Badge: up to level 30

this goes on in intervals of 5 levels all the way up to 8 Badges, at which point the level cap is removed.

The pokemon can spawn with different levels as well, take your Onix encounter as an example. Consider you having 1 badge meaning you could catch Onix up to level 25, you might encounter one that is level 30 or higher, meaning its too powerful. However there are also lower level Onix spawns as well, say you encounter a level 10 Onix instead meaning it would not be too powerful to catch.

You can obtained a Chipped Pot (as well as Cracked Pot) from the bargain merchants in Stow-on-Side, particularly the one who sells you an item. Serebii has a list of all the items you can obtain from them.

Unfortunately, it is not a guaranteed chance of finding one. The item the merchant sells changes every day, so you will need to check his inventory roughly every 24 hours (or after midnight, need to verify) to see if he is selling a Chipped Pot.