Is there a limit for iControl Rest API POST requests per minute?

The iControl REST framework does not impose a rate restriction, however, you may saturate the underlying system such as mcpd if you send too many requests too soon. You may need to consider throttling your requests. For configuring the entire box with full of objects, yes, AS3 would be a good choice (it's becoming popular).

Please also note that there is a restriction on a number of tokens per user. If you are not reusing the token (assuming you are using the Token Based Authentication) but generating one for each object, then you may get the token error (looks like this is not the case from your error message). The current limit is 100. This limit is not applicable to the 'admin'.