Hey Frisha. I use straw/hay bales for a large vegie garden area.
see here and here and here As you can see, the bales are not physically holding back the soil, just keeping it in place if it subsides. It will work as a retainer, i just have plenty of space and so keep the bales back a little.
You don't need to put rebar thru the bales. They will be very solid and will become more-so as they age.
The bales need to be replaced every year to 2 years, which is fine, as they go from being the sides to becoming the mulch or just being dug into the beds as you prep them for the next crop. They are a great additive for soil. The life which grows in and under them as they decompose is excellent for your soil.
If you want a bed with permanent
plants, you could still use them, just don't
plant right beside the bale. Leave about a foot if possible, to allow for a bit of subsidence. Ideally though, they are best for vegies and for a garden which is annual or seasonal..
They would be fine for both applications you have in mind, and for a temporary bed they are ideal.
The best thing to put into the beds is compost, however, it may not be available in the quantities you need. One thing for sure though, your existing soil should be utilised if possible, so have a look at it and see what it is like.
I can send you a detailed description of how to build up your soil if you like.