Memory Settings for SQL Server, SSAS, and SSRS
we are not using the mulitdimensional model, not tabular    


Do you mean NOT multi-dimensional ONLY TABULAR?

If so:
It's not best practice to install SSAS and SQL Server together on one server as the SSAS (Tabular) VertiPaq Engine works totally different than SQL Engine and it's (VertiPaq) is more memory intensive unless the models configured as DirectQuery mode. You could may have different VM for each of them if the license covered physical server cores  

If not so:
I don't think that is practical as you must select either option TABULAR/MULTI-DIMENSIONAL during SSAS Installation. if you mean, there is no use of either SSAS options, you could simply uninstall it from the server

Yes, it's recommended to configure maximum server memory setting when there are multiple instances on the same server (other applications on database server not recommended), depending on the instance workload you could set max server memory setting, leaving minimum ~4GB to OS. I have seen ~16GB memory (for SSRS dedicated server) sufficient to handle around 150 users requests

Consider following settings are same as maximum server memory in SQL Server


In SSRS: WorkingSetMaximum
In SSAS: TotalMemoryLimit