Understood the APs terminate on the locals, but the masters still do things like ARM, WIDS, config management and some data is upreported to the master. The 7030 has a max client count of 4096, so assuming 16 users per AP, that's a max reporting AP count of 256 APs (that's the genesis of the limits when we note scalability for masters).
In general though, 7000 controllers should be the master for other 7000 controllers, 7200s are masters for 7200s.
That said, there's no hard limit that prevents you from over-subscribing the 7030 as a master. So just be aware if there are issues with the master in terms of stability or performance, the likely culprit will be that the 7030 is overloaded from a pair of 7210s with maybe 1000 APs and more users than the master is able to support.