It's a good idea to get a variety of different skill sets on a Scrum team. The more self-sufficient the team is, the less you have to go outside for help with the design and implementation.
One of the key components of a productive Scrum team is balance: balance of skills, balance of personalities, balance of responsibilities, balance of workloads, and balance of power.
Agile teams require highly motivated people in order for the fast paced, iterative process to be effective. Many teams suffer from lack of motivation. In times like this, it's important to stay positive.
The Scrum Guide suggests having between three and nine team members to work on executing the sprint backlog.