Knitting technique

I have a shawl I knitted many years done similarly on two different sized knitting needles. The stitch was a bit more complicated than straight K1 P1. I seem to remember it was called blackberry stitch, and because of the uneven and large size of the needles, it came out loopy and lacy. I still have the pattern for it.

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This sounds like a great idea. I love it when people "freestyle" with knitting. Re the casting off, I would do the final row before casting off with both needles if the same gauge (the thinner one), thereby avoiding cast off problems.

Yes, do a tension square , but casting off could be a problem, cast off your tension square to see if it's too tight or too loopy.