There's nothing wrong with mix and matching cogs as needed. i've been doing this since cassettes became popular in the mid 80's and here's a few things i've learned.

1. If the lower cogs are riveted or screwed together the the rivets can grinded off and the screws can be removed with NO adverse consequences.

2. In many cases the first 2 high gear cogs can NOT be easily interchanged due the their built in spacers.

3. If NON matching cogs are swapped out i.e. a P for a Q or a Q for an R shifting quality will be diminished but is still going to be better than the old uniglide days.

Its easier and cheaper to start with a cassette that has the high gear cogs you need.