I can't speak for the scale out part, but for the rest, I think it really depends.
For the ports, I recommend any that are not in use and that your IT dept allows to be opened in the firewalls and that are preferably not in any reserved port lists. Default ports are likely going to be fine unless you have some need to change them.
As for the service accounts, which accounts can you live with if they get disabled? If you have one account per service, then if one gets locked out, you only lose new things that service may do. If all of them are the same account, you lose all of them. Where I work, we don't follow best practice with the accounts and it is one SQL account for all live SQL stuff (SSRS, SSIS, SQL Agent, and SQL Instance). The advantage is it is a single account we need to manage in AD. Downside is if it gets locked out, it locks it out on ALL of those services. For SQL Instance we've never had any issues, but the SQL Agent will fail to start new jobs. Offhand, I am not certain if Polybase needs to spawn anything new once it is running so it may not be impacted by a locked account.
If you want scale out, I am pretty sure you would want them to all have the same account. That way the permissions can be set up identically on all of the servers and it makes it nice and easy. Otherwise permissions are going to be painful to set up for each instance of it. But I'd check the official docs from MS to find that information out.