The t2.small will kick you in the pants in two spots, FWIW: compute and bandwidth. The first probably won't matter for a redis cluster, but the second could definitely hurt.
Amazon refuses to be pinned down on how small (it's "low to moderate" bandwidth on their instance matrix), but you will feel the pinch with extended usage.
If you've only got a gig of RAM available seems unlikely you could make any decent use of high bandwidth situations (or if you are, your cache is going to pretty much constantly evicted).
But still, you can hop on an m4.xlarge for only 150 a month. Get 14GB of available cache on 4 cores with high networking.
FWIW, redis is maybe an outlier here, but it's pretty stark. The prices are just so far out of reasonable range it feels like. Their postgres pricing seems more in line (assuming heroku's support is good, I have no experience with such).
Amazon refuses to be pinned down on how small (it's "low to moderate" bandwidth on their instance matrix), but you will feel the pinch with extended usage.
https://aws.amazon.com/ec2/instance-types/#instance-type-mat...