It's no problem to use the same master for multiple agent OSes, even Windows. In fact, if your Linux and Windows nodes both require some of the same modules, a single master for both will be more maintainable. The main thing you want to monitor is the total number of nodes vs. installing your master monolithically or as a split install.
We recommend moving to a split install once you manage in the range of 500 to 1500 nodes. More information about split install can be found here: https://docs.puppetlabs.com/pe/latest/installpesplit.html
Also make sure that you meet all of the system requirements, no matter what kind of installation you choose: https://docs.puppetlabs.com/pe/latest/installsystemrequirements.html
Oops, I just assumed that you are using PE and not FOSS. The instruction links I posted above apply to PE, and if you are using PE, you can also purchase on-site assistance through the Puppet Professional Services group. There is an overview of the offerings here: http://puppetlabs.com/services/professional-services