We benchmark against 2GB / 2 Core Linux VMs, whatever the RAM, we recommend multi-core servers for best performance.
I wouldn't recommend putting Redis and Tyk on the same server, Redis has high RAM overheads so the two will compete. Also, in cases of DR, you will want to be able to fail-over your redis hosts.
If you are using Redis Cluster, then having them on the same host could cause some weird scaling problems (you would need to add the new hosts to the redis cluster configuration somehow) if you are auto-scaling.
Let me know if that helps.