This is an IP networking question; you're looking for some information on how IP is configured and routes traffic. (There's nothing particularly specific to OS X Server and its DHCP server here; any DHCP server and any IP network will have the same limitations and options.)
The following assumes you're usually running 200-some DHCP hosts and often close to ~252 hosts.
With your existing /24 subnets (aka that 255.255.255.0 subnet mask), you can't (meaningfully) expand those DHCP pools; that /24 subnet allows host addresses from .1 to .254. The .0 and .255 addresses aren't available as host addresses.
You either need to use a larger subnet (eg: a /22 subnet) or you need multiple subnets and some additional routing between the subnets.
Adding subnets and spreading the load around can avoid overrunning the capacity of a local network; ~250 hosts can generate a substantial amount of network traffic, particularly on lower-bandwidth WiFi connections, or (when you have muiltiple WiFi devices) on the backhaul networks.
Using subnets in different private blocks isn't something I'd recommend. Stay in one private block if you can. Probably adding subnets in the 10/8 block here, given you're already in that block and given it's the biggest available private block.
And not all devices around can support a subnet other than a /24. Some widgets are limited to /24, unfortunately.
