Yes, you can.
The maximum number of concurrent connections is limited to 100 on a Type 0 instance, but no other limits are imposed. The “Max 25 rooms” is a rough estimation of the maximum number of rooms that can be created while keeping good performance in a typical game.
The actual number of rooms that can be created while keeping good performance depends on various factors such as the number of replicated objects in a scene. Exceeding an appropriate number of rooms may degrade your game’s performance and cause some problems with game play. So, Strix Cloud allows you to limit the maximum number of rooms per room server instance in your game titles. This limit is not a fixed value, and you can set any number that best fits your game. You can set a number larger than 25 for a Type 0 instance (and its default is 100 rooms). When the limit specified by you is reached, it becomes impossible to create more rooms.
By the way, other than the number of rooms, Strix Cloud provides up to 60GB of data per month for free of charge regardless of the instance specs, and the data exceeding the amount incurs data overage fees. Although you can use a Type 0 instance without registering a payment method, please be aware that the instance may be stopped when the amount of data transfer in a month reaches 60GB with no payment method set.