Can webMethod IS and Aris sit in one server?
This post is a bit outdated, but still worthy of a response.
Yes, you can install both products on the same server, there is no conflict in this regard. However, the decision to how to deploy ARIS or webMethods Integration server also needs to take into consideration how you intend to scale the products and support a robust reliable deployment.
ARIS is very good out of the box at scaling down to a small single server deployment but really shines well when you fan out its runnable components to achieve a HA, reliable and performance oriented architecture.
Also, the webMethods I.S is also equally capable of fanning out and distributing workloads via multiple instances on 1 server or multiple instances across multiple servers.
If your putting together a basic development environment, the only thing to consider is really is there enough CPU and memory available within reason to run both products. For example I have run both on my DELL i7 laptop with 16GB RAM. This is not a production style deployment, but good enough for to play around with.