Can you mix gigabit and 10/100 ethernet on the same network?
Solution 1:
Gigabit or 100Mbit (or 10Mbit) will be decided on a link by link basis, and it is possible to mix the two within a network. Each router/switch should automatically negotiate the fastest link speed (although sometimes it's better to specify this manually, if possible).
So, it makes sense to group all 100Mbit hardware with your existing 100Mbit switch; likewise for the Gigabit hardware and router. It will not slow down the Gigabit section of the network.