Cordyline australis often looks tatty at the end of winter, and may be killed in a hard winter (many were 2 years ago, even large mature ones). They also develop a trunk within 2 years and all the growth will be at the top. As you're growing them in pots, you can at least move them somewhere under cover or very sheltered over winter if it proves necessary.
You might want to consider Phormiums instead - these don't develop a trunk, but spread sideways, so after a couple of years or so, you'd need to turf them out of the pots, split and repot. They also can be killed in a seriously hard winter though.
Both plants don't need a lot of feeding, even in pots - an application of a general purpose fertiliser in spring turned into the compost at the top when you're topping up is all that's necessary.

As suggested Phormiums (look for the less vigorous varieties) will do fine in pots as long as you can find a sheltered spot to overwinter. On the other hand, Cordylines will quickly outgrow their pots to become trees. You will forever be potting them on or have to plant them in the garden.

Thank you for your input about grasses a further visit to the garden centre I think before I decide ,your comments are helpful