I have no need to use retools workflows - so currently i have deployed self hosted retool via google cloud run in a single container SERVICE_TYPE="MAIN_BACKEND,JOBS_RUNNER,DB_CONNECTOR_SERVICE"
Can I expect any downsides to not running the other services?
Those three services should be all you need if you don't intend to use Workflows.
There should not be any downsides from not running other services. 
Great thanks for confirming.
Are there any issues with scaling if they're all in a single instance? Can I for example scale horizontally to multiple instances with this setup?
Yes, there are 
JOBS_RUNNER can only have a single instance (as per the docs) it runs tasks like db migrations amongst other things. so multiple conmtainers with that service are a bad idea.
I would split that out if i needed to scale instances.
But i'm currently serving 3 users so not a huge issue to deal with yet