Indeed, XOD has its source open. You can find the source on GitHub (https://github.com/xodio/xod) and do whatever you want as long as it follows AGPL free license terms.
What’s for the limits, we set them on our cloud usage rate. Things like simulation program compilation are performed in the cloud, on our servers because it would be very hard for an average xoder to configure the necessary toolchain on the local machine. Our servers cost money, so we limit the usage. Soon it will be possible to sign up for a paid plan to have big limits.
Anyway, the limits are replenished continuously (not monthly, or even daily, but once per 10 minutes). See https://xod.io/settings/billing/ for the log.