Hi Sorcha
Yes they will interbreed.
I personally would not put them in your 5ft tank as they would get lost.
so best thing to do is to get only 1 pair.
If you want to breed them then you will be better off getting a smaller tank of around 10 litres in size with a tight fitting lid.
If you want cheaper killies you will need to join the New Zealand Killiefish Association. However the ones on trademe are a good price.
PM Alan if you want to join.
There are a few of us killiephiles here all the time.
We are more than happy to help out a newbie killie keeper