RE: Deploy commerce to the same environment as existing FO instance....is to possible?
Cloud hosted eCommerce can be setup via LCS by following Deploy a new e-commerce tenant.
If you want to setup eCommerce on the Dev VM then follow Set up a development environment. Doing so involves having Retail Server deployed locally on the Dev VM and it could be deployed by using both modes (mentioned here and here) - IIS and Self Hosted (note that Self-Hosted scale unit dev experience for eCommerce is supported starting 10.0.23). With Self Hosted however, there are limitations (obviously they will be present since Self Host flavor cuts/mocks a lot), for instance, payment flow doesn't work yet.
If you will want to use self-hosted RS with eCommerce then some additional changes might be needed in addition to what is mentioned in the above documentation:
1. Set MSDyn365Commerce_LOCAL_BASE_URL=true
in the .env file
2. Update RS's CORS setting to accept the RS's Url: Open the file C:\Program Files\Microsoft Dynamics 365\10.0\Commerce Scale Unit\Microsoft\RetailServerSelfHosted\Microsoft.Dynamics.Retail.RetailServerSelfHost.AspNetCore.dll.config in any editor, locate there AllowedOrigins and append the url, including correct port number, corresponding to your locally deployed eCommerce, so after your modifications that key's value should be similar (see highlighted changes) to:
<add key="AllowedOrigins" value="usnconeboxax1pos.cloud.onebox.dynamics.com;https:/.../usnconeboxax1ecom.cloud.onebox.dynamics.com;https://localhost:4000" />