Right now I'm learning piano using a 61-key Yamaha keyboard. The keys are lightweight and offer little resistance, however, it's the right size, weight, and price for my current limitations.
Can I continue to use this for the first six months or so of my piano education, or will this cause problems when I transition to a proper piano or keyboard with weighted keys?
This is exactly how I started, so my experience might be of interest in a small way.
I started with a Yamaha E333 61-key electronic keyboard. I'd never played a piano in my life before (that's not exaggeration), until I was 62. It took me many months of constant practice, on my own, to get to the stage where I could play simple pieces (one note on each hand, no chords), and I loved it. It taught me perseverance, still the most valuable lesson I have learned. I assumed that the transfer to a 'proper' keyboard would be uneventful and without difficulty, since my fingers would 'know' where to go, no matter what the keyboard (as long as the black keys weren't painted on, of course).
Then came the stage where I knew I needed a teacher, so I found one, and happily, I hit 'gold' first time. In my first session with her, she sat me down at a full-sized keyboard (a Clavinova, which she uses to 'transition' students brought up on small keyboard to prepare them for the grand), and asked me to play a piece I knew well, just by way of assessing where I was. So I began to play a little minuet by Rameau, which I'd been practising for months, and could play with my eyes close (literally).
Within five seconds, everything fell over. My fingers forgot where to go, and it took me three or four restarts to get through it to the end. I was completely unprepared for the effect of the difference in 'tactile feedback' and the big difference in apparent effort required to make a note sound. I knew then that I'd have to move up to something nearer a full-sized and more realistically 'weighted' keyboard. Within a fortnight, I had tried out several models, and my then budget settled on a DGX640 Yamaha, and the E333 was given to a friend. (I've since added a Clavinova CLP470, with an acoustic now in the plans.)
However, it didn't take long to get adjusted ; one of my first lessons, after all, had been the value of perseverance, so I knew not to be discouraged, and after a couple of months on the Clavinova, she put me on to the Steinway for my now weekly lessons.
I don't regret a second of the time I spent with the little Yamaha keyboard. I learned a lot with it, had a great deal of joy from it, it helped with my sight-reading, and it made me realise that this was what I wanted to do. It gave me a leg up to a pastime I can't get enough of, at an age way beyond what I thought would ever be possible.