1) Yes, you can put longer term, but your stamp duty will be higher. One of the ways is you put one year but the tenant has the option to continue renting with the agreed amount (same amount or increase by how many percent).

2) You can actually put more than that. But depend, if tenant will able to find cheaper one or places more convenient to them, they may move after the minimum period. The option will not cost anything to them.

It is good that you sign another agreement after the earlier one end, or else the tenant has the freedom to move out or not paying you because no more agreement bound.