0.25 are good for practicing as they are a nice thickness so not too fiddly.
Once you become confident you can happily chuck these in a drawer never to be seen again but while you're training don't stress.
Yes the finer lashes are better and I don't use 0.25.
But don't worry about that for now.
Just perfect your technique and then when you feel more confident move on to 0.20 or 0.15.
You'll love them.
But back to the original question, I think its fine to mix thicknesses if its only one size...so 0.15 and 0.20 are ok together but that does depend on the clients lashes.
You will learn with experience what works best.
I would say try not to complicate things while you are learning.
Stick to the same size/curl for now and also as a newbie I don't think you should use more that 2 or 3 lengths to begin with as you may get muddled and them they'll be uneven.
9,11,13 are good sizes to practice with, although I find most ladies don't need more than 11mm believe it or not