I would like to know whether there are any restrictions/rules when developing a product which is compatible with Google/Alexa or maybe even other hubs. Do I need to get some license or follow some rules?
Google Documentation includes a check list which has the ToS
Especially read the first item on the list on that page.
For Amazon you should probably start here:
And look at "Certification and Badging" section
As of know they don’t have any restrictions as they give you permission to tether with your television music player and etc , they don’t have any problem by accessing your own product through Alexa or google assistant., if you can make effective product you can do that and on other hand you are just going to pass the voice command to Alexa and Alexa should react to your own device that is the thing you have to take care so the choosing methodology and devices does the matter here , do more research on this and then start to develop your product