When I practiced scales and scale patterns, I was told by my teacher to use a variety of different "routes" through the 12 keys. For example, here are the routes I most frequently used to get through all 12 keys:
- go around the circle of fifths (e.g., C, G, D, A, etc.)
- go around the circle of fourths (e.g., C, F, Bb, Eb, etc.)
- go up by minor thirds (C, Eb, Gb, A then up a half step to Db, E, G, Bb, then up a half step to D, F, Ab, B)
- go up by tritones (e.g., C, Gb, Db, G, D, Ab, Eb, A, etc.)
There are other ways to get through all 12 keys--these are just the sequences I used and preferred. For example, you could go up major thirds, increasing by half steps (C, E, Ab, then Db, F, A, then D, F#, Bb, then Eb, G, B). Additionally, for every ascending pattern I've listed, you can play through the 12 keys using the same intervals but in descending form.
Variety is useful because it makes us more flexible. It gives our brain more avenues for accessing/retrieving our memory of a given scale (and muscle memory), which ultimately makes us quicker at retrieving the information. However, variety serves a more practical function: it presents a challenge which keeps us interested and willing to do something as monotonous as practicing scales in all 12 keys. So any amount of variety will be beneficial to your technique and motivation.
Regarding the different types of scales (major, natural minor, melodic minor, etc.), I would recommend sticking to one type and taking it through all 12 keys before moving to the next type of scale. For example, I would take the major scale through all 12 keys, then the melodic minor scale through all 12 keys (by a different route), then the harmonic minor, etc. This will emphasize the underlying structure and intervals which make up each type of scale. I think this is more beneficial than the alternative, playing C maj and A natural minor together. Playing C maj and A natural minor back-to-back will emphasize the fingering and modal relationships, which I think isn't worth focusing on because it the connection between the types of scales is straightforward and requires less practice to grasp.
Once you are comfortable with the different types of scales (major, melodic minor, etc.), it could be valuable to alternate between different types of scales as you go through the 12 keys. If it were me, though, this sort of practice would be secondary, and would only occur once I had mastered the different types of scales in all 12 keys.
- practice a single type of scale (major, melodic minor, etc.) at a time in all 12 keys
- go through the 12 keys using a variety of different routes and intervals
- once you are comfortable with the different types of scales, you can cycle through different types of scales as you cycle through the 12 keys