Is it a bad idea to mix integer types ?
It can be. As your example shows, when you mix integer types in arithmetic expressions, the language applies conversion rules, some of which are hard to remember.

Does it affect speed ?
I don't know, but I'm willing to bet you'd be hard-pressed to measure any difference.

Is it considered a "bad practice" ?
The Go programming language forbids it, but only so that they didn't have to define the implicit conversion rules. My approach is to do whatever -Wall -Wextra -pedantic -Werror will allow.

Also, is it terrible to use 64-bit integers only ?
No, it is not terrible. Your 64-bit machine sees the world mostly as 64-bit integers.