I have observed that some coffees have quite a bitterness immediately after roasting... and that it settles after some time. I would attribute this to the off gassing period where there is still a lot of CO2 being generated by the beans. Under pressure of extraction, the CO2 is pressurised into the brewed liquid creating carbonic acid... which is naturally bitter!

I think they should go pretty well together. 5:1 is a good ratio to start with, but it may be worth doing two separate roasts initially just to understand how each type behaves in the roaster and what flavour it brings to the cup. Then when you taste the blend, you should be able to pick what each type brings to the cup! Best wishes on your roasting! Let us know how it goes!